Characterizing Global Archaeocyathan Reef Decline in the Early Cambrian: Evidence from Nevada and China

UNLV Retrospective Theses & Dissertations

1-1-2005

Characterizing global archaeocyathan reef decline in the Early Cambrian: Evidence from Nevada and China

Melissa Hicks University of Nevada, Las Vegas

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Repository Citation Hicks, Melissa, "Characterizing global archaeocyathan reef decline in the Early Cambrian: Evidence from Nevada and China" (2005). UNLV Retrospective Theses & Dissertations. 2658. http://dx.doi.org/10.25669/4dcj-qx5g

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This Dissertation has been accepted for inclusion in UNLV Retrospective Theses & Dissertations by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. CHARACTERIZING GLOBAL ARCHAEOCYATHAN REEF DECLINE IN THE

EARLY CAMBRIAN: EVIDENCE FROM NEVADA AND CHINA

Melissa Hicks

Bachelor of Science, Geology Juniata College 1999

Masters of Science, Geoscience University of Nevada, Las Vegas 2001

A dissertation submitted in partial fiilfillment of the requirements for the

Doctor of Philosophy Degree in Geoscience Department of Geoscience College of Sciences

Graduate College University of Nevada, Las Vegas May 2006

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Hicks, Melissa

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The Dissertation prepared by

______M elissa Hicks

Entitled

Characterizing the Global Archaeocyathan. Reef Decline in the Earlv

Cambrian; Evidence from Nevada and China______

is approved m partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Geoscience ______

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

Characterizing Global Archaeocyathan Reefs Decline in the Early Cambrian: Evidence from Nevada and China

Melissa Hicks

Dr. Stephen M. Rowland, Examination Committee Chair Professor of Geoscience University of Nevada, Las Vegas

This study presents new data on the sedimentology of the northern Yangtze

Platform, the ô^^C stratigraphy of South China and Nevada, and the decline and virtual

extinction of archaeocyaths. Paleoecological, sedimentological, and chemostratigraphic

data were collected from multiple localities that span from the mid-Early Cambrian of

Nevada (Poleta Formation) and China (Xiannudong Formation) to the late-Early

Cambrian of Nevada (Harkless Formation) and China (Tianheban Formation).

Facies represented in the Xiannudong Formation describe a Bahamas-type

platform for the Yangtze Platform with a static and negative ô^^C record. All four

formations analyzed show a static ô^^C record that varies from slightly negative to

slightly positive. This is unexpected because stasis is not observed in the composite ô^^C

record of the Siberian Platform, suggesting that our data may represent intrabasinal Ô^^C

records and not global.

Another important feature I observed in the Xiannudong Formation is a faunal

changeover from predominantly regular-type archaeocyaths in the lower Xiannudong

Formation to predominantly irregular-type archaeocyaths in the upper part. This

111

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. changeover is also seen between the Poleta Formation and the Harkless Formation in

Nevada.

Finally, I analyzed the physical changes in archaeocyathan skeletons over time. A

progressive thinning of skeletal thickness was expected, however a distinct trend of

skeletal thickening was observed. Skeletal thinning was expected due to a lowered

carbonate saturation state driven by rising atmospheric CO 2 . To explain thickening, I

propose that irregular archaeocyaths contained photosymbionts, which are documented to

counteract a lower carbonate saturation state. Irregular archaeocyaths, which inherently

contain more elements in their intervallum, and hence, more soft tissue, could house more

endosymbionts than regulars. Therefore, regulars declined due to the lack of abundant

photosymbionts, while irregulars flourished.

Currently, there is no definitive evidence to support our hypothesis that the

ultimate extinction of archaeocyaths was due to increased sea surface temperatures due to

the greenhouse warming effect of increased pCOz Prolonged temperatures would have

increased thermal stress on the archaeocyaths until some threshold was reached in which

even irregulars could not survive. This scenario may have implications for modem reef

reactions to anthropogenic CO 2 and greenhouse warming.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ABSTRACT...... iii

LIST OF FIGURES...... vii

LIST OF TABLES...... ix

ACKNOWLEDGMENTS...... x

CHAPTER 1 INTRODUCTION...... 1 References ...... 4

CHAPTER 2 EARLY CAMBRIAN MICROBIAL REEFS, ARCHAEOCYATHAN INTER-REEF COMMUNITIES, AND ASSOCIATED FACIES OF THE YANGTZE PLATFORM...... 6 Abstract...... 6 Introduction ...... 8 Methods ...... 13 Stratigraphy of the Xiannudong Formation and Paleoenvironmental Interpretation ...... 14 Paleoenvironmental Synthesis ...... 48 Implications for the Yangtze Platform ...... 51 Acknowledgments ...... 51 References ...... 52

CHAPTER 3 CHEMOSTRATIGRAPHIC ANALYSIS OF EARLY CAMBRIAN MICROBIAL REEFS, ARCHAEOCYATHAN REEFS, AND ASSOCIATED FACIES IN NEVADA AND SOUTH CHINA: A SURPRISING STORY OF STASIS...... 57 Abstract...... 57 Introduction ...... 59 Methods ...... 63 Stratigraphy and Sedimentology ...... 64 Biostratigraphy...... 72 Chemostratigraphic Analysis...... 74 Conclusions ...... 78 Acknowledgments ...... 79 References...... 80

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4 SYSTEMATIC AND MORPHOLOGIC TRENDS IN ARCHAEOCYATHS THROUGH THE EARLY CAMBRIAN...... 90 Abstract...... 90 Introduction ...... 92 Methods ...... 95 Archaeocyaths...... 99 Systematic and Morphologic Variation...... 102 Mechanisms for Systematic Change ...... 108 Summary and Conclusions ...... 117 Acknowledgments ...... 119 References ...... 119

CHAPTER 5 SUMMARY OF MAJOR RESULTS ...... 128 References ...... 131

APPENDIX 1...... CD-ROM

VITA...... 134

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

CHAPTER 2 Figure 2.1 Study Area Map ...... 9 Figure 2.2 Stratigraphie Correlation Diagram...... 10 Figure 2.3 Three Measured Sections of the Xiannudong Formation ...... 15 Figure 2.4 Expanded Fucheng Section Stratigraphie Column ...... 16 Figure 2.5 Outcrop and Photomicrographs of the Fucheng Section ...... 19 Figure 2.6 Outcrop and Photomicrograph of the Fucheng Section ...... 21 Figure 2.7 Outcrop of Microbial Reefs of the Fucheng Section ...... 22 Figure 2.8 Photomicrograph of Thrombolitic Texture in the Xinchao Section ...... 25 Figure 2.9 Photomicrographs of float-rudstone of the Fucheng Section ...... 29 Figure 2.10 Photomicrographs of float-rudstone of the Fucheng Section ...... 31 Figure 2 .11 Outcrop and Photomicrographs of Unit L of the Fucheng Section ...... 32 Figure 2.12 Photomicrograph of Archaeocyath of the Xinchao Section ...... 36 Figure 2.13 Block Diagram of the Paleoecology of the Xiannudong Formation at the Fucheng Section ...... 37 Figure 2.14 Outcrop Photograph and Rose Diagram of Oolite Cross-bedding of the Fucheng Section ...... 39 Figure 2.15 Outcrop and Photomicrograph of the Silty/sandy Micrite of the Fucheng Section ...... 42 Figure 2.16 Photomicrograph of Girvcmella in the Oncolite Bed of the Xinchao Section ...... 45 Figure 2.17 Outcrop Photograph of the Oncolite Bed of the Shatan Section ...... 46 Figure 2.18 Block Diagram of the Paleoecology of the Xiannudong Formation...... 49

CHAPTER 3 Figure 3.1 Study Area Maps ...... 60 Figure 3.2 Stratigraphie Correlation Diagram...... 61 Figure 3.3 Composite ô^^C Profile for the Siberian Platform...... 62 Figure 3.4 Measured Sections and Corresponding ô^^C Profiles for the Xiannudong and Lower Poleta Formations ...... 65 Figure 3.5 Measured Sections and Corresponding ô^^C Profiles for the Tianheban and Harkless Formations ...... 69

Vll

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4 Figure 4.1 Spindle Diagram with the Numbers of Archaeocyathan Genera in the Early Cambrian ...... 93 Figure 4.2 Study Area Maps ...... 96 Figure 4.3 Stratigraphie Correlation Diagram...... 97 Figure 4.4 Photomicrographs of Irregular and Regular Archaeocyaths...... 100 Figure 4.5 Global Frequency of Archaeocyathan Genera Graphs ...... 103 Figure 4.6 Relative Biomass of Regular and Irregular Archaeocyaths...... 104 Figure 4.7 Thickness Means of Archaeocyaths ...... 109 Figure 4.8 Thickness Means of Archaeocyaths ...... I l l Figure 4.8 GEOCARB Model of Atmospheric CO 2 ...... 113 Figure 4.9 Diagram Illustrating C02 Interactions ...... 114

Vlll

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

CHAPTER 2 Table 2.1 Point Counts of Float-rudstone samples from the Fucheng Section 27 Table 2.2 Genera of Archaeocyaths present within the Float-rudstone from the Fucheng Section ...... 28 Table 2.3 Point Counts of the float-rudstone and oncolite from the Xinchao Section ...... 34

CHAPTER 3 Table 3.1 ô'^C and 5*^0 values from the Poleta Formation ...... 84 Table 3.2 ô’^C and ô’Ô values from the Fucheng Section of the Xiannudong Formation ...... 86 Table 3.3 ô'^C and 5**0 values from the Xinchao Section of the Xiannudong Formation ...... 87 Table 3.4 S’^C and ô'Ô values from the Tianheban Formation ...... 88 Table 3.5 ô’^C and ô'Ô values from the Harkless Formation ...... 89

CHAPTER 4 Table 4.1 Thickness Measurements of Archaeocyaths ...... 106 Table 4.2 Statistical Measurements of Archaeocyaths ...... 110

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS

This dissertation was only completed because I had both funding and support

from Professors, collogues, and friends. For fimding my project, I would like to thank

the Geology Department’s Bernada French Scholarship, American Museum of Natural

History, Institute for Cambrian Studies, Geological Society of America, Paleontological

Society, Arizona-Nevada Academy of Sciences, and the Graduate Student Professional

Association.

I would like to recognize and thank those on my committees. My committee

Djibouti (my comps committee) consisting of Michael Wells, Gene Smith, Matt Lachniet,

and Steve Rowland served well to mess with my brain and watch me squirm, while

asking fair questions. My dissertation committee Omega consisting of Andrew Hanson,

Terry Spell, Steve Rowland, Ganqing Jiang, and Peter Starweather were extremely

helpful and also challenging. I must bring special attention to Steve, my advisor, who

was my field partner in China and in Nevada, read through my terrible first drafts with

patience, and helped me cultivate my knowledge of geology, while at the same time

reminding me the “limits of my knowledge”. Françoise Debrenne must be acknowledged

and whole-heartedly thanked for her involvement in my understanding of archaeocyathan

taxonomy and for allowing me to live in her home for a month. I also want to thank Jay

Kaufman who worked with me to run my chemostratigraphic analyses at the University

of Maryland’s Stable Isotope Lab. and for the many discussions on the data.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. There are a number of other professors (who did not make it onto one of my

committees) who helped me in various ways and they are too numerous to state here, but

I want to point out in particular, Russell Shapiro, Rod Metcalf, Tom Anderson, Kim

Johnson, Brett McLaurin, and Cathy Snelson. I learned a lot from the UNLV department,

mostly that you can never have too many people involved in a dissertation.

During my 6.5 years at UNLV, I met a lot of other graduate students not just from

Geology, but also from Physics, Anthropology, and Kinesiology. These guys made my

Wednesdays and BOBs more enjoyable and reminded me to have frm too. Two people

stand out in my mind as the most important geologists and friends I will ever know,

Joseph Kula and Amy Brock. These two helped me to survive my dissertation, in which

during times of panic and stress they were always there to lend an ear or a helping hand.

The best part of my dissertation was getting to spend more time with them. To Joe, who

put up with me during my times of utter despair and unnecessary drama, I thank you the

most, you truly are the love of my life and the reason I have not taken a sharp

archaeocyath to my wrists.

Finally, I thank my parents and sister, who always helped when needed, never

judged, and allowed me to choose my own path (even with purple hair). You never said

no to a new book, you never allowed me to goof off in school, you were tough and stem,

but you always allowed me to live my own life. I am who I am because you were a great

family, weird, but great. I dedicate this dissertation to you.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1

INTRODUCTION

During an eleven-million-year interval in the Early Cambrian Epoch (521-510

Ma), archaeocyaths, an extinct class of calcareous sponges, formed a reef-building

consortium with calcium carbonate-secreting microorganisms (calcimicrobes). This reef-

building consortium of archaeocyaths and calcimicrobes was short lived, but it was

impressive because of the rapid diversification and global distribution that archaeocyaths

achieved. Archaeocyaths evolved on the Siberian platform -521 Ma, globally radiated

by 518 Ma, and virtually went extinct -510 Ma, except for rare, non-reef-building,

putative Middle and Late Cambrian archaeocyaths found in Antarctica (Debrenne et al.,

1984; Wood, et al., 1992).

Archaeocyathan-calcimicrobe reefs grew as isolated patch reefs, as well as

laterally extensive compound reefs. Some preserved compound reefs are as large as 100

m in thickness (Rowland and Shapiro, 2002; Rowland and Hicks, 2004). Based on the

presence of primary fibrous cements and the associated sediments, these reefs typically

formed on high energy, shallow, carbonate platforms and ramps (Mathews, 1974). A

wide variety of reef-dwelling organisms also are associated with these reefs and include

trilobites, brachiopods, hyoliths, bivalved arthropods, chancellorids, salterellids,

echinoderms, and coralomorphs.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Archaeocyaths occupy an interesting point in Earth’s history as the first

multicellular animals to actively participate in reef-building and the first reef-building

organisms to become extinct (Rowland and Hicks, 2004). This ecologically and

structurally complex reef ecosystem collapsed and disappeared at the end of the Early

Cambrian, followed by a forty-million-year interval (Middle Cambrian through the Early

Ordovician) during which reefs built by multicellular animals were essentially non

existent on Earth. The enigma surrounding the decline and virtual extinction of

archaeocyaths and the 40-million year gap in metazoan-reef-building, one of the longest

gaps ever recorded, continues to puzzle paleontologists and geologists alike. The

mechanisms that have been proposed for the archaeocyathan decline vary from climatic

to chemical, and testing these mechanisms has proven to be problematic (Rowland and

Shapiro, 2002).

The list of potential culprits for archaeocyathan extinction is long, while our

ability to rigorously test each hypothesis is limited. Climate models suggest that the

levels of atmospheric CO2 rose very rapidly during the Early Cambrian (Berner, 1991,

1994, 1997; Bemer and Kothavala, 2001) accompanied by high atmospheric and sea

surface temperatures (Karhu and Epstein, 1986), but the error bars associated with these

models are large. It has also been inferred that during the Early Cambrian there was a

change in seawater chemistry fi-om aragonite-precipitating conditions to calcite

precipitating conditions (Sandberg, 1983, Stanley and Hardie, 1999). Furthermore, sea

level changes, such as the Sinsk Event (a transgression that introduced anoxic waters onto

the carbonate platform) and the Hawke Bay Regression, also have been suggested as a

prime cause of archaeocyath extinction (Wood, 1999).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In order to sort out the true mechanisms behind the decline of archaeocyathan

reefs, I studied archaeocyaths and reefs from the peak reef-building interval (mid-Early

Cambrian) to the extinction (end-Early Cambrian). Data were collected from field

locations in western Nevada, southern Shaanxi, northern Sichuan, and northern Hubei

provinces, China. These localities were chosen because: (1) they contain putative coeval

archaeocyath-bearing units that span the mid-Early Cambrian to the late-Early Cambrian,

(2) they were widely separated in the Early Cambrian (as they are today), and (3) the

paleoecology of two of these units (Poleta and Harkless formations) had previously been

studied (Rowland, 1978; Hicks, 2001).

Each of the three papers included in this dissertation describes a different aspect

of this study: (1) the paleoecology and sedimentology of the Chinese archaeocyath-

bearing sections, (2) the chemo stratigraphie analyses, and (3) the changes in

archaeocyaths over time. In order to better characterize the decline of archaeocyaths, the

following three hypotheses were tested: (1) the generic richness of archaeocyaths

declined from the mid-Early Cambrian to the end-Early Cambrian, (2) the percent of

branching archaeocyaths decreased from the mid-Early Cambrian to the end-Early

Cambrian, and (3) the abundance of irregulars increased from the mid-Early Cambrian to

the end-Early Cambrian. In addition to these, ô^^C isotope chemostratigraphy was used

to test the hypothesis that the Lower Poleta Formation is correlative with the Xiannudong

Formation and the Harkless Formation is correlative with the Tianheban Formation.

This dissertation by no means completes research into the cause of the

archaeocyath extinction and the metazoan-reef-free period following, but it produces new

models for organism reactions to environmental change, which can be used to analyze

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. both ancient and modem reef builders. Furthermore, it tests the ability for

chemo stratigraphy to correlate strata globally and its use in interpreting

paleoenvironments.

References

Bemer, R , 1991, A model for atmospheric CO 2 over Phanerozoic time: American

Joumal of Science, v. 291, p. 339-376.

Berner, R., 1994, GEOCARB II: a revised model of atmospheric CO 2 over Phanerozoic

time: American Joumal of Science, v. 294, p. 56-91.

Berner, R , 1997, The rise of plants and their effect on weathering and atmospheric CO 2 :

Science, v. 276, p. 544-546.

Bemer, R., Kothavala, Z , 2001, GEOCARB IQ: a revised model of atmospheric CO 2

over Phanerozoic time: American Joumal of Science, v. 301, p. 182-204.

Debrenne, F , Rozanov, A.Yu., and Weber, G.F., 1984, Upper Cambrian Archaeocyatha

from Antarctica: Geological Magazine, v. 121, p. 291-299.

Hicks, M., 2001, Paleoecology of the upper Harkless archaeocyathan reefs in Esmeralda

County, Nevada [masters thesis]: Las Vegas, University of Nevada, Las Vegas

159p.

Karhu, J., and Epstein, S., 1986, The implication of the oxygen isotope records in

coexisting cherts and phosphates: Geochimica et Cosmochimica Acta, v.50,

p. 1745-1756.

Matthews, R.K., 1974, A process approach to diagenesis of reefs and reef associated

Limestones, in Laporte, L.F., ed.. Reefs in time and space: Tulsa, Oklahoma,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Society of Economie Paleontologists and Mineralogists, p. 234-254.

Rowland, S. M., 1978, Environmental stratigraphy of the Lower Member of the Poleta

Formation (Lower Cambrian), Esmeralda County, Nevada [Ph.D. thesis]: Santa

Cruz, University of California, 107p.

Rowland, S.M. and Gangloff, R A , 1988, Structure and Paleoecology of Lower

Cambrian Reefs: PALAIOS, v. 3, p. 111-135.

Rowland, S.M., and Shapiro, R S , 2002, Reef patterns and environmental influences in

the Cambrian and earliest Ordovician: SEPM Special Publications, v. 72,

p. 95-128.

Rowland, S., and Hicks, M., 2004, The Early Cambrian experiment in reef-building by

metazoans: in Lipps, J.H., and Waggoner, B.M., eds., Neoproterozoic—Cambrian

Biological Revolutions: Paleontological Society Papers, v. 10, p. 107-130.

Sandberg, F A , 1983, An oscillating trend in Phanerozoic non-skeletal carbonate

mineralogy: Nature, v. 305, p. 19-22.

Stanley, S.M., and Hardie, L.A, 1999, Hypercalcification: paleontology links plate

tectonics and geochemistry to sedimentology: GSA TODAY, v. 9, p. 1-7.

Wood, R., Evens, K.R., Zhuravlev, A.Yu., 1992, A new post-early Cambrian

archaeocyath from Antarctica: Geological Magazine, v. 129, p. 491-495.

Wood, R , 1999, Reef Evolution: Oxford University Press, New York, 414p.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2

EARLY CAMBRIAN MICROBIAL REEFS, ARCHAEOCYATHAN INTER-REEF

COMMUNITIES, AND ASSOCIATED FACIES OF THE YANGTZE PLATFORM

Abstract

A sedimentological and paleoecological study of part of the northern Yangtze

Platform of China was conducted. Three sections of the Early Cambrian

(Atdabanian/Botomian Stage) Xiannudong Formation where studied, and reveal that

stromatolite and thrombolite reefs flourished in close association with an inter-reef

community of archaeocyaths and other organisms. Adjacent to the reef environment

were migrating ooid shoals, oncoids, peloids, and silty/sandy lime mud.

No archaeocyathan reefs were observed in any of the measured sections, but

irregular archaeocyaths were found in some microbial reefs, and archaeocyaths form the

bulk of the debris in float-rudstone that was deposited between microbial reefs. We

interpret the archaeocyath-rich float-rudstone to represent a storm-churned inter-reef

deposit. Typically, ‘regular’ type archaeocyaths dominated the float-rudstone, while

‘irregular’ type archaeocyaths were found in abundance dwelling within the microbial

reefs. This trend changed upsection as ‘irregular’ type archaeocyaths dominated both the

float-rudstone and in the microbial reefs.

The oncolite and peloidal mudstone are interpreted as representing a quiet water,

lagoonal environment created by the presence of the microbial reefs and ooid shoals.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. During large storms material from the ooid shoals or microbial reefs would wash into the

lagoon. The silty/sandy micrite represents a quiet water setting, which could either be a

deep-subtidal environment or a restricted lagoonal environment.

The facies associations of these three sections, along with other published data

indicate that the Yangtze Platform, at least during the Early and Middle Cambrian, was a

carbonate platform and equatorially located.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction

This paper is part of an overarching study on the decline and virtual extinction of

archaeocyathan reefs during the Early Cambrian. In order to characterize the decline of

the archaeocyathan reef ecosystem, we incorporated data from localities that were

globally distributed during the Early Cambrian. Included in this data set, are samples

from the Xiannudong Formation (mid-Early Cambrian) from the northern part of the

Yangtze Platform. The purpose of this paper is to (1) describe the Early Cambrian

microbial reefs and associated facies from the Xiannudong Formation, (2) interpret their

paleoenvironments, and (3) make inferences concerning the paleogeography of the

Yangtze Platform. Prior to this study, archaeocyathan-built reefs were described from

multiple exposures of the Xiannudong Formation by Yuan and Zhang (1981), Qin and

Yuan (1984), Zhang (1989), Ye et al. (1997), and Yuan et al. (2001) from outcrops in

southern Shaanxi Province and northern Sichuan Province (Figure 2.1), but neither the

paleoecology nor sedimentology of the reefs had been studied in detail. New fieldwork

and subsequent laboratory research conducted between 2002-2004 call for a re-evaluation

of the reefs in the Xiannudong Formation.

In the absence of internationally recognized stage terminology for the

Cambrian Period, we use the widely used Siberian Platform nomenclature (Figure 2.2)

for this paper. The Xiannudong Formation is late Atdabanian to early Botomian in age.

The Botomian, in particular, was the stage of the Early Cambrian during which

archaeocyathan generic diversity was at its maximum and reef-building by archaeocyaths

and associated calcibionts was at its peak. Because the three sections of the Xiannudong

Formation are well exposed, they provide an opportunity to determine the three-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bewing China

'107 E Shaanxi Province Nani tana ^chan^—^ Sichuan Province Chengdu

Shatan Fucheng ¥ * ‘ Kunming Xinchao Nanjiang

15km Figure 2.1. Location map of China illustrating the boundaries of the Yangtze Platform (Li et ai., 1995, 1996) and the locations of the three measured sections (stars) of this study. F = Fucheng section; S = Shatan section; and X = Xinchao section. Inset is a larger-scale map of the field area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Laurentian Chinese Stages Siberian Chinese Strata Great Basin Stages and N. Sichuan-S. Shaanxi and Zones Stages Strata Zones W est East o> Mule Spring Kongming- Megapalaeolenus Tianheban Zone dong Saline Wiley I Fm. (equalivalent) c Fm. Ç 0 0) " c O) q> CD Palaeolenus 3 c CO I CO Zone I c 511 Ma Harkless n II Î Shipai Ê S Yanwanbian Fm. Fm. CO 0> O If Drepanuroides QI Fm. CD O ) Zone I c Ç 0 I la Yunnanaspisr E Yiliangella Zone (D m C3> Poleta CO Malungia Zone 5 1 8 M a 3 Xiannudong Fm. LU 0) CO Liangshui- Fm. &-C Yunnano- c Nevadella jing cephalus CO Zone I E Fm. Zone c Ç 0 c Campito Ysunyi- Ic s o Fm. II discus CO Fallotaspis Guojiaba Zone Fm. Parabadiella 1 n.II 52QMa Figure 2.2. Stratigraphie correlation of formations included in this study. Formations are placed in Stages and biozones based on biostratigraphy as described in the text (Rowland 1978; Yuan and Zhang 1981; Qin and Yuan 1984; Zhang 1989; Ye et al. 1997; Yuan et al. 2001; Zhu et al. 2001; Zhuravlev and Riding 2001; Hollingsworth 2004).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dimensional relationships of the reef horizons and associated facies. This study

complements a previous study of coeval strata on the southern part of the Yangtze

Platform by Debrenne and Jiang (1989), which documented facies relationships similar to

those of the Xiannudong Formation. This new look at old reefs illustrates the dynamic

environmental and evolutionary changes that occurred during the Early Cambrian,

including cyclic environmental changes, relationships between archaeocyaths and

microbial reefs, and evolutionary changes in the archaeocyaths themselves.

Age of the Xiannudong Formation

Figure 2.2 summarizes the age of the Xiannudong Formation, subjacent and

supeijacent formations, and correlative strata of the northern Yangtze Platform and

coeval strata from the Great Basin, U.S.A. The age of the Xiannudong Formation is

based on the presence of the trilobites Malungia granulose, Micangshania gracilis (Yuan

and Zhang 1981; Zhang 1989), Yunnanocephalus, Zhenbaspis, Eoredlichia, and

Kuanyangia (Yuan et al. 2001). Yuan and Zhang (1981), Qin and Yuan (1984), and

Zhang (1989) all place the Xiannudong Formation straddling two trilobite biozones,

Malungia and Yunnanaspis-Yiliangella, which are in the Tsanglangpu (Canglangpuan)

Stage. Ye et al. (1997) and Yuan et al. (2001) place the Xiaimudong Formation in three

different biozones, Eoredlichia-Wutingaspis, Malungia, and Yunnanaspis-

Yiliangella, which straddle the upper Qiongzhusi (Chiungchussan) Stage and the lower

Tsanglangpu (Canglangpuan) Stage. The Qiongzhusi/Tsanglangpu stages correlate to the

upper Atdabanian to lower Botomian on the Siberian Platform and to the upper

Montezuman Stage of Laurentia (Zhuravlev and Riding 2001; Hollingsworth 2004)

(Figure 2.2).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Geological setting

The position and tectonic history of the Yangtze Platform during the Proterozoic

and Early Cambrian are not well understood. Some reconstructions consider the Yangtze

Platform to be part of the South China block (attached to Cathaysia block or the “mobile

belts”) in the Early Cambrian (Nie, 1991; Li et al., 1996; Zhao et al., 1996; Huang et al.,

2000), while others infer that it was an isolated platform (Li et al., 1995; Xu et al., 1996;

Wang and Li, 2003). Furthermore, the inferred paleolatitude of the Yangtze Platform

varies from between 20° and 30° north (Nie, 1991; Li et al., 1995), to greater than 30°

north (Li et al., 1996), to lying on the equator (Zhao et al., 1996; Huang et al., 2000;

Scotese, 2001), to lying between 15° and 30° south of the equator (Xu et al., 1996).

Rifting of the South China Block (consisting of the Yangtze Platform and the

Cathaysia Block) from Rodinia is hypothesized to have occurred in three rifting phases,

beginning in the Neoproterozoic (-820 Ma) (Wang and Li, 2003). Rifts formed along the

western (Kangdian Basin) and the southeastern margins (Nanhua Basin) of the South

China Block (Li et al., 1999; Wang and Li, 2003). During the formation of the Nanhua

basin, the South China Block became isolated from the Cathaysia Block (Wang and Li,

2003). Rifting ended at approximately 690 Ma, and was followed by thermal subsidence.

In the Neoproterozoic, the Yangtze Platform was tilted to the southeast (Wang, 1986;

Korsch et al., 1991). This resulted in the development of different depositional

environments in the western and eastern portions of the Yangtze Platform, and a thicker

accumulation of strata is observed in the east.

In the Yangtze Gorges area, west of Yichang (Figure 2.1), the Cambrian is

approximately 1,300 meters thick, while in the western portion of the platform the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cambrian is less than 500 meters thick. In the northern Sichuan/southern Shaanxi

provinces the Cambrian strata include the Guojiaba, Xiannudong, Yangwangbian, and

Kongmingdong formations, which display a general theme of cyclic sedimentation with

silty/sandy micrite, shale and siltstone, interbedded with limestone and dolostone.

Methods

One month of fieldwork was completed during the summer of 2002, where we

measured, described, and collected fi"om the three sections of the Xiannudong Formation

(Figure 2.1). Collected samples were cut for 3 x 2 inch thin sections. These thin sections

were used to identify archaeocyathan genera, micro-fossil identification, and for point

counts. Species-level identification of archaeocyaths is problematic under the best of

circumstances. Because most of the archaeocyaths in this study were transported and

abraded, we have identified them to the generic level, or, in some instances, to the ordinal

level. In this paper we distinguish between ‘regular’ and ‘irregular’ archaeocyaths. This

distinction was formally thought to reflect class-level evolutionary significance (Hill,

1972). However, archaeocyaths are now placed within the Phylum Porifera as an extinct

class of aspiculate sponges (Debrenne et al., 2002). Taxonomic revisions within the

Archaeocyatha taxon have left the regulars and irregulars as morphologically distinct

groups, but they are no longer recognized as separate clades.

Almost all percentages discussed in this paper are the result of point counts on

thin sections. Point Counts were preformed on the archaeocyathan-float-rudstone,

oncolite, and oolite using a Hacker Instrument, James Swift point counter. For each

slide, 400 points were counted, with a four-increment spacing between each point.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carbonate terminology in this paper follows Folk (1962) and Dunham (1962) and the

modified scheme by Embry and Kilo van (1971).

Stratigraphy of the Xiannudong Formation and Paleoenvironmental Interpretation

The Xiannudong stratotype section is a road cut near the village of Shatan in

northern Sichuan, north of the town of Nanjiang (Figure 2.1). At Shatan, the Xiannudong

Formation is approximately 110 m thick and contains multiple, thick intervals of oolite,

microbial reefs, oncolite, and thinly-bedded, quartz-rich, silty/sandy micrite (Figure 2.3).

The microbial reefs typically contain a clotted and/or laminated fabric, but they do not

contain any identifiable microbial entity. Two archaeocyath-bearing units occur in the

upper part of the Shatan section. In both of these units, the archaeocyath-bearing units

are within oncolite-bearing beds. The archaeocyath skeletons are not in situ and are

broken and abraded.

The Xiannudong Formation at the Fucheng section is 67.3 meters thick and is

located near the village of Fucheng (Figure 2.1). It is underlain by the Guojiaba

Formation and overlain by the Yanwanbian Formation (Yuan et al. 2001), all of which

are structurally deformed into a broad anticline. In this section, we divided the

Xiaimudong Formation into thirteen stratigraphie units (units A-M) (Figure 2.4).

The Xinchao section of the Xiannudong Formation is a road cut near the village

ofXinchao in Tongjiang County, northern Sichuan Province (Figure 2.1). Here, the

Xiannudong Formation is 113 meters thick, which is slightly thicker than at Shatan and

much thicker than at Fucheng (Figure 2.3). The Xiannudong in the Xinchao section

consists of multiple intervals of thinly bedded, peloidal, silty/sandy micrite, oncolite, and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Base of Top of Base of Yanwanbian Fm. Q-j 3 jexposure Yanwanbian Fm.

R C #6

covered

R C #5

R C # 4

;C#3

R C # 2

OmlillllH Bill RC#'I Top of Guojiaba Fm. Oolite Silty/sandy micrite

Microbial reek Archaeocyatti float-rudstone m Oncolite

Peloidal wackestone/packstone

Grainstone Figure 2.3. The three measured sections of the Xiannudong Formation in this study. Section localities are on Figure 1. Dashed lines indicate gradational boundaries. Thicknesses are in meters. Reef Complexes (RC) are numbered for the 0 m 0 m_ Fucheng Section. See figure 4 Top of G uojiaba Fm. Top"6f Guojiaba for details of the Fucheng Section. Fm.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD Q.O C Q.g

■D CD

C/) Top of exposure C/)

O olite

Silty/sandy micrite 8 m Microbial reefs (O' Float-rudstone

G rain sto n e

R e e f @ 'Regular' archaeocyath C om plex 'Irregular' archaeocyath 4 3 & 3" 10- Thrombolite CD A exposure ■DCD J|[f^Strom atolites O Q. Gold C o a Fossil hash 3o â > "O B raohiopod o Echinoderm plates

Cross-bedded oolite Q.CD Horizontal burrows C om plex Trilobite

■D om plex Figure 2.4. Measured section of CD K cont. Xiannudong Formation at the C/) C/) Fucheng section. The section is 67.3 meters thick. Tie-lines connect points of continuation. R e e f C o m p le x , 1 Top of Guojiaba Fm. F cont. oolite. Only two intervals contain archaeocyaths, which occur as constituents in a float-

rudstone that is very similar to the archaeocyath-bearing float-rudstone in the Fucheng

section. The multiple cycles of stromatolitic and thrombolitic reefs with micrite and

silty/sandy micrite that are conspicuous at Fucheng do not occur at Xinchao.

In the following sections, we describe each facies of the Xiannudong Formation,

with emphasis on the Fucheng section where the most significant facies shifts were

observed.

Microbial Reef Facies

We interpret these stromatolites and thrombolites as reefs based on Flügel and

Kiessling’s (2002) definition, which defines a reef as a laterally confined, biogenic

structure that was constructed by sessile, benthic organisms, exhibits topographic relief,

and rigidity. The microbial reefs in this study typically exhibit most of the above-

mentioned features, which will be described below.

As illustrated in Figure 2.4, the microbial reef intervals within the Xiannudong

Formation in the Fucheng section vary in thickness from several decimeters to several

meters and are laterally extensive. The material surrounding the microbial reefs is

archaeocyathan-rich floatstone to rudstone. The archaeocyaths within the float-rudstone

are subrounded to subangular clasts that reflect variable amounts of abrasion due to

transport. The archaeocyathan-rich float-rudstone is described in greater detail in the

next section.

The lowest reef complex. Reef Complex 1 (Unit A), within the Fucheng section is

1.5 m thick, with several discrete domal and columnar stromatolites that vary in size

(Figure 2.4). These domal and columnar stromatohtes contain in-situ irregular

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. archaeocyaths. Archaeocyaths comprise 3.7 percent of the microbial reefs in Reef

Complex 1. The genera present are both irregulars, Protopharetra and Graphoscyphia

(Figures 2.5A, B, C).

The presence of archaeocyaths as dwellers within microbial reefs is not unusual

during the Atdabanian/Botomian. Thrombolitic and stromatolitic reefs with low

abundance archaeocyaths (<10%) are present in South Australia (James and Gravestock

(1990) and Mongolia (Kruse et al., 1996). However, at those localities, archaeocyath-

built reefs are also present, either in a different environment or in a different stratigraphie

position. The Xiannudong sections are unusual for the Atdabanian/Botomian in the sense

that although archaeocyaths were present, they were not involved in reef construction,

even though a variety of depositional environments are represented.

As with most stromatolites, no identifiable microbial entity is present within Reef

Complex 1. Aggrading neomorphism and dolomitization have obscured the primary

microbial fabric in the samples. Burrows are present in both the stromatolites and in the

associated archaeocyathan float-rudstone. Based on the lack of flanking bedding within

the archaeocyathan-rich float-rudstone, we infer that these microbial reefs probably

attained only modest relief on the seafloor.

Reef Complexes 2-6 (Units D, G, I, K, and M) (Figure 2.4) are lithologically and

paleontologically similar to Reef Complex 1. Differences occur in the presence of a

more thrombolitic texture within some units, especially in Reef Complexes 3 and 4, and

in the percentage of archaeocyaths within the microbial reefs. In outcrop, the boundaries

between the thrombolitic and stromatolitic fabrics are not conspicuous. Thrombolitic

fabrics occur at the base of Reef Complexes 3 and 5 with stromatolitic fabrics overlying

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I

Figure 2.5. (A) Reef Complex 1 (Unit A) in the Fucheng Formation showing some of the individual stromatolitic and thrombolitic microbial boundstone (M and black lines). The float-rudstone is the more resistant material that is present between the microbial reefs. (B) Photomicrograph (X-polars) of transverse and longitudinal section of the archaeocyath sp. (P) in stromatolite (S). (C) Photomicrograph (X-polars) of the poorly preserved archaeocyath Grcphoscyphia sp. in stromatolite reef.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. them. However, this is not typical; elsewhere thrombolitic fabrics are observed in the

middle and near the top of a reef complex, overlying stromatolitic fabrics.

In Reef Complex 2 (Unit D, 1.6-m thick), Protopharetra sp. is the only

identifiable archaeocyath within the stromatolitic reefs (no thrombolitic reefs are present

in this unit); it constitutes approximately 1% of the reef by volume. Upsection in Reef

Complex 3 (Unit G, 1.1-m thick), thrombolitic reefs occur at the base of the unit, with

stromatolites dominating the upper portion. The stromatolites form distinct columns,

some of which are taller than 30 cm. Flanking beds of float-rudstone are visible, which

indicates that these columns had topographic relief on the seafloor (Figure 2.6A). No

archaeocyaths are present within these particular microbial reefs, but poorly preserved

cocci (IRenalcis) are preserved (Figure 2.6B).

Reef Complex 4 (Unit I, 6.1-m thick) contains the first appearance of a ‘regular’

archaeocyath in the microbial reefs, however irregular archaeocyaths still dominate;

archaeocyaths constitute 2.2 % of the reef by volume. These archaeocyaths are poorly

preserved, but a few can be identified as Protopharetra sp. and Inessocyathus sp. Both

stromatolites and thrombolites are present in this unit, but with no conspicuous pattern in

their distribution. In some places, stromatolites grade upward into a thrombolitic fabric

in a seemingly continuous reef, while elsewhere the opposite occurs (Figure 2.7).

Reef Complex 5 (Unit K) is the thickest interval of microbial reefs (approximately

12.5 m thick). As in Reef Complex 3, thrombolitic reefs form the base of this unit with

stromatolitic reefs forming the bulk of the upper portion. Within this reef complex,

poorly preserved cocci are present, along with sponge spicules, but no archaeocyaths.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I

Figure 2.6. (A) Outcrop photograph of columnar stromatolites (S) from the upper portion of Reef Complex 3 (Unit G). Scouring of the stromatolite is observed with float-rudstone in the scoured pockets. Float-rudstone occurs between the stromatolites. (B) Photomicrograph (X-polars) of poorly preserved cocci, IRenalcis, (arrows) from thrombolitic reefs of lower part of Reef Complex 3.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .A

Figure 2.7. Outcrop photograph illustrating the difficulting of discerning between stromatolite and thrombolites reefs in outcrop, in which both are present here. Photograph is of Reef Complex 4 in the Fucheng section. Staff is 1 meter.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The uppermost reef complex, Reef Complex 6 (Unit M, 14.4-m thick), contains

stromatolite reefs with poorly preserved ‘irregular’ and a few possible ‘regular’

archaeocyaths. The poor preservation of the ‘irregulars’ allows for only tentative

identification of Protopharetra sp. and Graphoscyphia sp. in this reef complex. The

stromatolites are interspersed with completely dolomitized grainstone, ooid-archaeocyath

float-rudstone, and oolite.

All of the Reef Complexes (1-6) in the Fucheng section contain microbial reefs

with scoured peripheral boundaries (Figure 2.6A). These scour cavities appear to be the

product of wave scour, and they are typically filled with a high percentage of ooids and

fossil hash, including archaeocyathan debris. The presence of these scour-and-fill

features is interpreted as the result of periodic storm events eroded the peripheries of the

microbial reefs.

In the Xinchao section, the microbial reef facies is very similar to the reef facies

of the Fucheng section, although at Xinchao the microbial reef facies comprises a much

smaller component of the section. Only unique characteristics of the reefs are described

below. Whereas distinctive columns and hemispherical masses occur in the microbial

reefs at Fucheng, the Xinchao microbial reefs are massive, nondescript micrite in

outcrop. In the lowest interval of microbial reefs, at about 50 m (Figure 2.3),

archaeocyathan float-rudstone pods interfinger with highly neomorphosed micrite. This

aggrading neomorphic spar occurs in small clusters, producing a clotted texture in thin

section. Several samples contain cocci microfossils, and therefore, the majority of the

microbial reefs in the lowest interval of the Xinchao section appear to be thrombolitic.

We speculate that neomorphism preferentially altered those areas of clotted micrite, while

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. leaving the microbially mediated clots relatively unaltered. Xenotopic to idiotopic

dolomite also hinders the interpretation of the primary fabric in these micritic samples

(Figure 2.8). Archaeocyaths belonging to the irregular Order Archaeocyathina are

present in the thrombolitic reefs, but, due to poor preservation, we could not identify

them below the ordinal level.

In the Shatan section, the microbial reef interval varies from conspicuous

microbial clotted fabrics to subtle clotted and laminated fabrics of both thrombolites and

stromatolites. No microbes or calcimicrobes are preserved. As in the Xinchao section,

the microbial reefs at Shatan do not form columns or hemispherical masses, such as those

of the Fucheng section. Instead this facies appears as massive, nondescript micrite and

nodular-bedded micrite with some rudimentary laminae. No archaeocyaths are

conspicuous in outcrop or thin section in the Shatan reef samples.

Microbial reef paleoenvironment

In the Fucheng section, we interpret the stromatolitic and thrombolitic reefs to

have formed on a shallow carbonate platform. The lack of exposure features, along with

the presence of scour-and-fill features on the periphery of some of the microbial reefs,

indicates a highly energetic, shallow-subtidal environment. We interpret the

interfingering of facies to record occasional major storm events during which ooid shoals

abruptly buried the adjacent microbial reefs, only to be subsequently recolonized by the

microbial reef community.

Neither the Xinchao section nor the Shatan section contain the well-defined

microbial reefs with conspicuous topographic relief that are so conspicuous in the

Fucheng section. This indicates that environmental conditions were not optimal for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.8. Photomicrograph (X-polars) of neomorphosed micrite (N) between clots of microbial cocci (M) in the microbial reef faces of the Xinchao section.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. microbial reef growth in either of these sections. In section 4 of this paper, we provide a

paleoevironmental synthesis.

Archaeocyathan-rich Float-rudstone Facies

In sections of the sediment adjacent to the microbial reefs in the Fucheng section,

large (>2mm) archaeocyathan skeletal clasts, microbial roll-ups, oncoids, ooids,

brachiopod valves, trilobite carapaces, Chancellorid spicules, hyolith cones, echinoderm

plates, and a multitude of unidentifiable fossil fi'agments are present (Table 2.1).

Dolomite also is present and varies from euhedral to anhedral crystals that form

idiotropic to xenotropic mosaics. The taxonomic mix of archaeocyaths, as well as the

percentages of archaeocyaths and other fossil constituents, varies from one float-rudstone

interval to the other. Table 2.2 lists the genera present in each interval of float-rudstone

in the Fucheng section. In the following discussion, the abundance of each constituent is

given as the average percent by volume. Only archaeocyath percentages and any unique

differences will be discussed for each of the Fucheng float-rudstone units.

The float-rudstone that occurs in association with Reef Complex 1 at Fucheng

contains the typical assemblage and percentages of constituents observed in the float-

rudstone. These constituents are as follows: archaeocyaths (15.5%), peloids (3-5.7%),

trilobite fragments (0.7%), other unidentified fossil hash (3.7%), ooids (<1%), and very

fine to medium silt-size quartz (1.7-17.2%) (Table 2.1). The matrix, which constitutes

the remaining volume, is a varying mix of micrite and calcite cement. Three genera of

archaeocyaths were found in the fioat-rudestone of Reef Complex 1 (Figures 2.9A, B).

Most of the archaeocyath fragments range in size from 3.0-11.0 mm, and show moderate

rounding. Roll-up stromatolites and mm-size domal stromatolites are present in this unit

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD O Q. C 8 Q.

■D CD

C/) C/)

8 ci' Table 2.1. Point counts in percents from thin sections of samples of float-rudstone and oolite from the Fucheng section. Float-Rudstone Samde No. Archaeocvath Micrite Peloid Soar-sm Soar-med/le Microbial Trilobite Dolomite Quartz Ooid Other Fossil

3-7-7 CRCn 15.2 43.7 3.0 3.5 5.7 20.0 0.7 0.0 1.7 0.0 3.0 3 l-7-7b (RCn 15.5 48.2 5.7 2.0 7.5 0.0 0.0 0.0 17.2 0.0 3.7 3" CD lOa-7-7 (RC21 12.7 21.0 1.0 4.2 12.5 0.0 2.0 29.5 9.7 0.0 7.2 4-7-8 (-RC3) 10.2 62.0 1.5 2.7 2.7 0.0 0.5 10.7 0.0 5.7 3.7 CD ■ D 8-7-8 (RC4) 8.0 56.5 1.5 1.7 20,5 2.5 0.0 5.5 0.5 0.0 3.2 O 8-7-8b fRC4t 5.0 42.2 1.2 5.7 9.7 0.0 0.0 22.2 0.0 0.0 13.7 Q. C 5-7-9 CRCS-) 11.2 61.2 10.0 2.7 1.2 0.0 0.0 1.5 4.0 0.5 7.5 a O 3-7-10 fRCÔ'l'l 9.6 60.4 1.2 3.5 0.0 0.0 0.0 19.2 1.2 0.0 5.4 3 4-7-10 (■RC6') 16.7 21.2 8.7 10.7 6.5 0.2 0.2 19.7 8.0 4.5 3.2 ■ D O Oolite 6-7-8b ('RC41 0.0 14.5 3.5 30.0 2.5 4.2 0.0 3.2 0.0 38.5 3.5 CD Q.

■ D CD

C/) C/) ::d ■DCD I I

CO 3o'

8 eg- Table 2.2. Genera of archaeocyaths present within float-rudstone between the microbial reef complexes in the Fucheng Section. 3 Reef ComDlex (Unit) RC #1 (Unit A) RC#2(UnitD) RC #3 U nit G) RC #4 U nit I) RC #5 U nit K) UnitL RC#6UnitM) CD Coscimcvathus XX XX X X Clathricoscinus XX X Taylorcvathus XX Conannulofuneia X ■DCD Inessocvathus X X <4 Rasetticvathus X X CI X g Rudanulus o K) Erismacoscims X 3 00 ■D Pilodicoscinus X S ^ Protopharetra X X X & "9 Graphoscvphia X X of Chansicvathus â Dictyocvathus X cO %

C/) o' 3 Figure 2.9. Archaeocyathan skeletal fragments in the float- rudstone of Reef Complex 1 in the Fucheng section. (A) Photomicrograph (X-polars) of a transported skeleton of Clathricoscinus sp. (C) with a stromatolite growing on it (S). (B) Photomicrograph (X-polars) of a transported skeleton of Coscinocyathus sp.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. only, and they occasionally encrust archaeocyath fragments (Figure 2.9A). Also, this

particular unit of float-rudstone contains abundant Chancelloria spicules that, although

found in some of the other units, are particularly abundant here.

The next interval of float-rudstone, associated with Reef Complex 2, contains

only one genus of archaeocyath, which is either Clathricoscinus or Coscinocyathus. The

float-rudsone does not contain any Chancellorid spicules, but it does contain well-

rounded glauconite, which is observed only in this interval. Reef Complex 3 contains

float-rudstone that contains three different genera of regular archaeocyaths (Table 2.2).

The float-rudstone associated with Reef Complex 4 contains the most diverse assemblage

of archaeocyaths of all the reef complexes (Figures 2.10A, B, C and Table 2.2), and also

has a conspicuous increase in the abundance of hyoliths. Reef Complex 5 is the thickest

unit in the Fucheng section (Figure 2.4). This reef complex contains the only sponge

spicules observed in this study. These are multiaxon spicules that are completely

recrystallized into blocky calcite; they occur in both the float-rudstone and the adjacent

microbial reefs. It is unknown whether these spicules were originally siliceous or

calcareous.

Unit L, which is predominately silty micrite, contains a cobble-size clast of

archaeocyath float-rudstone (Figure 2.11 A), of which archaeocyathan skeletal fragments

comprise 6.7% by volume. Based on the well-rounded nature of the cobble, we infer that

it was at least partially lithified prior to transport. The archaeocyaths in the float-

rudstone cobble were fairly well preserved, consisting of a very diverse assemblage of

regular archaeocyaths (Figures 2.1 IB, C, D, E, F and Table 2.2).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.0 mm

Figure 2.10. Photomicrographs (X-polars) of'regular* archaeocyaths from Reef Complex 4 in the Fucheng Section. (A) Oblique transverse cut through Conanrmlofungia sp. in float-rudstone. (B) Longitudinal cut through two Inessocyathus sp. in lens of oolite with patches of dolomite. (C) Transverse cut through Tc^lorcyathus sp. (T) in float-rudstone containing hyoliths (H) and brachiopods (B) among other fossil hash.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.0 mm

1.0 mm

: "%.,

1.0 mm 1.0 mm

Figure 2.11. Outcrop and photomicrographs (X-polars) of a clast of archaeocyath-rich float-rudstone within the silty/sandy micrite of Unit L. (A) Outcrop photo of cobble in silty/sandy micrite. (B) Transverse cut through Rasetticyathus sp. (C) Transverse cut through Dictyocyathus sp. (D) Transverse cut through two Pilodicoscinus sp. (E) Transverse cut i.Ojnm I through a portion of Rudanulus sp. (F) Oblique longitudinal cut through ?Erismacoscinus sp. (G) Longitudinal cut through Changicyathus sp.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reef Complex 6, in the uppermost unit at Fucheng (Unit M), shows a sharp

change in the volumetric dominance of archaeocyaths from regular to irregular (Table

2.2). The irregular archaeocyaths Protopharetra and Graphoscyphia dominate, with only

a few regulars of the genus Inessocyathus. This change from regular dominance to

irregular dominance is not unexpected within this part of the Early Cambrian, but it has

never before been documented within a single stratigraphie section. Younger Early

Cambrian archaeocyathan reefs (Toyonian Stage) contain a majority of irregular-type

archaeocyaths with rare regular-type archaeocyaths or none at all (James and KJappa,

1983; Debrenne et al., 1991; Hicks, 2001). We also observed this faunal changeover

from regular-dominated to irregular-dominated at the other two sections of the

Xiannudong Formation.

Only two intervals in the Xinchao section contain the archaeocyath dominated

float-rudstone facies (Figure 2.3). As observed at Fucheng, regular archaeocyaths

dominate the lower bed of float-rudstone (at 50 m); the same archaeocyathan genera are

present in this bed at Xinchao as occur in Units I and L of the Fucheng Section. These

include, Conannulofungia, Coscinocyathus, Rasetticyathus, Clathricoscinus,

ITaylorcyathus, and Pilodicoscinus. The irregulars in this interval are Graphyscyphia

and Protopharetra. These are all variably abraded. Fine-sand-size quartz occurs within

the float-rustone and averages 1% of the constituents. Other constituents of the lower

archaeocyathan-bearing unit are ooids, trilobites, echinoderms, peloids, and grapestones

(Table 2.3).

The upper interval within the Xinchao section, which contains conspicuous

archaeocyaths, occurs 111 m to 113 m above the base of the Xiannudong Formation

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD O Q. C 8 Q.

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C/) C/)

8 ci'

Table 2.3, Point counts in percents from thin sections of samples of float-rudstone and oncolite from the Xinchao section Float-Rudstone Sample No. Archaeocvath Micrite Peloid Calcite-sm Calcite-med/lg Microbial Trilobite Dolomite Quartz Ooid Other Fossil 3 5-7-11 fUAU) 20.0 32.7 1.7 3.5 15.2 0.0 0.0 9.7 8.2 4.0 4.0 3" CD 6b-7-ll rUAC) 2.5 61.0 2.0 6.0 3.7 0.0 0.0 7.2 2.0 15.2 0.2 7b-7-ll rUAUl 6.4 27.0 6.6 8.2 5.1 0.9 0.3 19.7 8.5 11.8 5.4 ■DCD 9b-7-ll ŒAU1 5.7 16.5 1.0 8.5 3.2 0.0 0.0 59.5 0.2 0.0 5.2 O 11-7-11 (XAUl 13.0 28.7 3.2 7.2 1.7 0.0 0.0 42.2 1.0 1.7 0.9 CQ. a o U) 3 ■D Oncolite O 4-7-11 0.2 51.0 0.0 19.0 15.0 8.5 0.0 5.7 0.7 0.0 0.0

Q.CD

■D CD

C/) C/) (Figure 2.3). That interval consists of a small amount of microbially mediated material,

and abundant silty/sandy micrite (medium to coarse sand-size quartz), with abundant

ooids, peloid, archaeocyaths, and fragments of trilobites and echinoderms. The

archaeocyaths are all irregulars, including ISpirillicyathus (Figure 2.12), Protopharetra,

and 1Graphoscyphia. These archaeocyaths are all relatively whole, with Graphoscyphia

occurring as a large, branching colony. Even though the archaeocyathan debris contains

large, branching specimens, including some with delicate exothecal outgrowths, these

archaeocyaths do not appear to be in-situ.

The Shatan section does not contain any archaeocyath-bearing float-rudstone; all

archaeocyathan skeletal fragments occur as nuclei of oncoids.

Archaeocvathan float-rudstone paleoenvironment

We interpret this facies to represent an inter-reef community of archaeocyaths and

other benthic organisms, such as trilobites and echinoderms (Figure 2.13). The

archaeocyaths in this inter-reef community were not forming reefs of their own, but lived

as individuals in clusters on the seafloor. Although they are not preserved in situ, the

presence of intact, branched forms and delicate exothecal outgrowths leads us to

conclude that they were not transported any significant distances. The microbial reefs

provided shelter from wave action, except during large storm events. These large storms

would rip-up the archaeocyaths and other organisms, tumble them, and then redeposit

them between the microbial reefs.

Oolite Facies

Of the three Xiannudong sections examined in this study, the Fucheng section

contains the least amount of oolite, while the Shatan section contains more oolite than

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.12. Photomicrograph (X-polars) of ?Spirillicyathus sp. in upper archaeocyath-bearing unit in the Xinchao section.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. # # © © © © © © © ©

è © regular

Stromatolite

Figure 2.13. Block diagram illustrating a "snap-shot" in time of the Fucheng section of the Xiannudong Formation.

Reproduced witfi permission of the copyright owner. Further reproduction prohibited without permission. any other lithology (Figure 2.3). In the Fucheng section, the ooids typically range in

diameter from 0.5-1.0 mm; some are recrystallized with saddle dolomite, while others are

partially dissolved and/or partially deformed. A sharp, wavy boundary occurs between

the microbial reefs of Reef Complex 1 and the first major occurrence of oolite. Unit B.

The basal portion of Unit B oolite contains trough cross-stratification with westward-

dipping planar cross-stratification at its top.

At Fucheng, Reef Complexes 4, 5, and 6 contain isolated to interfingering beds of

oolite associated with the microbial reef complexes. Unit I oolite does not appear to be

cross-bedded as observed in Unit B. It does contain several sizable (> 8mm), transported

regular archaeocyathan skeletal fragments of Inessocyathus and either Clathricoscinus or

Cosinocyathus. In contrast, the oolite beds in Unit M are conspicuously cross-bedded.

Furthermore, in Unit M, several thin beds of oolite interfinger with microbial boundstone

and also contain transported archaeocyathan debris. Unlike unit I, which contains only

regular archaeocyath debris. Unit M contains only irregular archaeocyath debris. Most of

the archaeocyaths within the oolite are too poorly preserved to be identified to the generic

level, but all are recognizable as irregulars. The few that can be classified to generic

level art Protopharetra and Graphoscyphia. Toward the upper part of Unit M, microbial

reefs disappear and cross-bedded oolite dominates (Figure 2.14A). Paleocurrent analysis

shows a bimodal, NNE-SSW, paleocurrent (Figure 2.14B).

In the Xinchao section, oolite occurs as one massive bed in the middle of the

section and as thinner interbeds within the uppermost archaeocyathan-bearing unit

(Figure 2.3). In the massive oolite bed, ooids range in diameter from 0.25-0.65 mm and

show varying degrees of dissolution and dolomitization; they are slightly flattened

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.14. Well defined cross-bedding in the oolite beds of Unit M. (A) Outcrop photo of cross-bedded oolite. (B) Rose diagram illustrating bimodial paleocurrent direction; n = 10.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. parallel to bedding. On weathered surfaces, weak cross-bedding was observed, but it was

not distinct enough to measure. In the uppermost archaeocyath-bearing unit, the ooids

(0.5-0.75 mm) are poorly preserved. Many are completely dissolved or dolomitized.

The Shatan section contains a massive amount of oolite that, for the most part, is

contained within the lower to middle part of the section (Figure 2.3). The ooids range in

diameter from 0.25 to 1 mm. Most show replacement by dolomite, but many display a

fibrous, radiating microstructure. Some oolite beds contain rudimentary laminations,

ripples or wavy bedding, and cross-bedding. Vertical burrows (3-5 cm in length) occur in

these beds, sometimes obscuring the sedimentary structures.

Oolite paleoenvironment

We interpret the oolite to represent migrating ooid shoals. Similar to modem ooid

shoals of the Bahamas carbonate bank (Purdy, 1963), these shoals formed in shallow,

high-energy marine waters, in particular shallow subtidal above normal wave base

(Purdy, 1963). The ooid shoal facies occurred adjacent to the microbial reefs and the

archaeocyath inter-reef community. Inter-fingering of the oolite and microbial reefs

mostly records shifts of the ooid shoals during storms. During large storms, the

migrating ooid shoals would bury adjacent microbial reefs and inter-reef communities,

which would sometimes reestablish themselves at a later time.

Silty/Sandy Micrite Facies

This facies occupies a significant portion of the Fucheng section, but it is rare in

the other two sections (Figure 2.3). Six intervals of silty/sandy micrite, with varying

thicknesses, occur in the Xiannudong Formation at Fucheng: units C, E, F, H, J, and L

(Figure 2.4). This facies always contains abundant horizontal burrows, but, with one

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. exception, no body fossils. The exception is Unit L, which contains the archaeocyathan

float-rudstone cobble described in Section 2.2. In general, the bioturbation is at a low

ichnofacies level of 1 or 2 on the scale developed by Droser and Bottjer (1988).

Differences do exist among these units, with variations including the following:

(1) the thickness of individual beds, (2) degree of irregular, wavy bedding, (3) amount of

bioturbation, and (4) amount of silt-size to fme-sand-size quartz. However, all units

contain neomorphic spar and are partially dolomitized, which causes great difficulties in

interpreting the paleoenvironment of this facies. Units C, E, F, J, and L are all very thinly

and irregularly-laminated (mm-thick laminae). Silt-size quartz occurs throughout units

C, F, and J (1-3%), which also contain numerous horizontal burrows with fine sand-size

quartz back-filling (Figures 2.15A, B). Unit E contains the coarsest material: fine sand to

coarse silt (0.05-0.15 mm)(5%) occurring not only in the burrows but also in some of the

wavy laminae. Unit L contains the most fine-grained material: silts occurring in rare

compacted horizontal burrows and sparsely throughout the micrite (3%). All of the

detrital quartz grains are subangular to subrounded. In contrast to the mm-scale laminae

of the other units. Unit H contains beds that grade from thin (mm-scale) to thick

(decimeter scale).

Silty/sandv micrite paleoenvironment

The silty/sandy micrite with horizontal burrows, mm-scale laminae, and wavy

bedding is difficult to interpret paleoenvironmentally. The presence of mm-scale laminae

indicates that it was a lower-energy environment than the environment of the microbial

reefs and oolite facies described in previous sections. The general absence of body

fossils indicates that the environment was generally inhospitable for organisms, with the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.15. (A) Outcrop photograph of silty/sandy micrite at Fucheng. Staff is 1 meter. (B) Photomicrograph (X-polars) of a horizontal burrow in silty micrite with neomorphic calcite spar. Note the preferential detrital quartz fill in the burrow.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. exception of sofl-bodied, horizontal burrowers. We consider two different hypotheses:

(1) the silty/sandy micrite represents deposition in a deep subtidal zone at or just below

storm wave base, or (2) this facies represents a lagoonal environment with restricted

circulation. The first hypothesis is based on the presence of horizontal burrows, lack of

body fossils, and fine lamination in the micrite (Wilson, 1975). The deep-subtidal

interpretation requires several deepening events to have occurred across the carbonate

platform. These deepening events could have been caused by either sea-level rise or

increased subsidence of the platform. We recognize several problems associated with the

deep-subtidal hypothesis: (1) sea level needs to have risen rapidly in order to drown the

microbial reefs, (2) evidence for a mechanism to cause the rapid sea level rise is lacking,

and (3) the silty/sandy micrite facies is conspicuous only in the Fucheng section. If this

facies represented an abrupt rise in sea level, then one might expect to see it

synchronously occurring in all three sections.

The restricted-lagoon hypothesis is based on the close association of the

silty/sandy micrite with the microbial reefs and inter-reef facies, including the presence

of a decimeter-scale, rounded clast of inter-reef-community float-rudstone within the

silty/sandy micrite in Unit L (Figures 2.4, 2.11). However, the restricted-lagoon

hypothesis is also problematic in that we lack a mechanism that would force repeated,

abrupt changes from a high-energy environment to a very low-energy, restricted

environment and back again. Each of these hypotheses for the paleoenvironment of the

silty/sandy micrite facies has its merits and problems, but the fine laminations and

horizontal burrows are inferred to indicate a low-energy environment.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Oncolite Facies

Two units within the Xinchao section contain oncolite, rare archaeocyaths,

trilobites, echinoderms, unidentified spar-filled circular fossils, possible brachiopods,

peloids, and very-fine to fme-sand-size quartz. The relative abundances of various

constituents within a sample from the lower oncolite are tabulated in Table 2.3. The

oncolite within the lower unit contains conspicuous Girvanella, a filamentous

calcimicrobe, which forms clusters and lenses (Figure 2.16). The oncoids are either not

well formed or are poorly preserved; they occur as distinctive, laminated, subcircular

bodies. The matrix consists of aggrading neomorphic spar, xenotropic to idiotopic

dolomite, and subrounded to rounded detrital quartz.

The upper oncolite unit contains more fossil debris than does the lower unit, as

well as better-developed oncoids. Archaeocyaths in this unit are all irregular and are very

poorly preserved. As with the lower unit, aggrading neomorphic spar and xenotopic to

idiotopic dolomite compose the matrix.

As in the Xinchao section, the Shatan section also contains oncolite intervals

(Figure 2.3), containing poorly preserved archaeocyaths. Although these transported

archaeocyaths are very large, the outer walls are completely abraded. Only a few

archaeocyaths could be identified to generic level. The lower oncolite unit (-88- 100 m)

contains archaeocyaths that are both regular (llnessocyathus sp.) and irregular

{Protopharetra sp ), which occur in subcircular, faintly microbially mediated, highly

bioturbated micrite to pitted oncolite (Figure 2.17). Archaeocyath percentages for this

unit only were calculated from slabs, using ArcView software with spatial analyst;

percentages range from 9.2 to 18.2% of the rock volume. Within the upper oncolite bed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.16. Photomicrograph (X-polars) of Girvanella (arrows) found within an oncoid from the lower oncoid-bearing micritic beds of the Xinchao Section.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.17. Outcrop photograph from the Shatan Section of pitted, oncolite bed. Oncolites are well to poorly formed and some samples contain well- preserved nuclei.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (-106-108 m) of the Xiannudong Formation at Shatan, irregular {Protopharetra sp, and

IGraphoscyphia sp.) archaeocyaths dominate as transported debris in silty micrite with

rudimentary laminae. With volumetric ranges from 31.8 to 42.6%, this unit contains a

considerably higher percentage of archaeocyaths than the lower unit.

Oncolite paleoenvironment

Oncolites typically form in low-energy environments with intermittent moderate

to high-energy fluxes (Wilson, 1975). We interpret the oncolite beds in the Xinchao and

Shatan sections to be shallow water, low-energy areas, in particular a lagoon.

Peloidal, Silty Micrite Facies

This facies is present only in the Xinchao and Shatan sections. It is especially

thick at Xinchao, forming the thickest unit in this section (Figure 2.3). Peloids typically

occur as pods (ranging from 10-40%) within an overall silty micritic matrix. Trilobite,

echinoderm, and other unidentifiable fossil fragments are present sporadically throughout

the matrix. Bioturbation does occur, but it is not as distinctive as in the Fucheng and

Shatan sections because the burrows are not infilled with coarser detrital quartz. Similar

to the silty/sandy micrite, the bioturbation in this facies is at a low ichnofacies level of 1

or 2 scale of Droser and Bottjer (1988). As with all of the rocks discussed in this paper,

dolomitization is widespread.

The most noteworthy characteristic of these massive peloidal, silty micrite beds is

the occurance of well-preserved Girvanella. Girvanella is present as patches within the

micritic matrix; it does not form distinct laminae or oncoids.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. At the very top of the Shatan section is a two-meter-thick, pink, peloidal

grainstone. The peloids are all well-rounded with no internal fabric. No cross-bedding

was observed in the unit, and no other fossils are present.

Peloidal. siltv micrite paleoenvironment

At both the Xinchao and Shatan sections, the intervals of peloidal, silty micrite

represent a quieter water setting than the microbial reefs, float-rudstone, oolite, and

oncolite represent (Wilson, 1975). The lack of body fossils in the peloid-rich silty micrite

indicates an environment that was not very favorable to organisms. However, the

presence of bioturbation indicates that some organisms were inhabiting or at least

feeding, but were not preserved (sofl-bodied organisms). The peloidal, silty micrite

represents a restricted lagoon in which lime mud and peloids were deposited and in which

soft-bodied organisms burrowed and feed.

Paleoenvironmental Synthesis

The northern margin of the Yangtze Platform, as represented in the three

Xiannudong Formation sections, was a shallow-water carbonate platform that

encompassed approximately 90 km^. The facies described above range from high-

energy, shallow subtidal to low-energy, lagoonal. We interpret changes in lithologies to

indicate lateral shifts in adjacent facies caused by storm events and recolonization of new

habitat by microbialite-reef-building organisms and associated inter-reef-dwelling

organisms (Figure 2.18).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. archaeocyathan inter-reef sitty/sandy micrite Fucheng community

Shatan

microbial reefs

Xinchao Figure 2.18. Schematic block diagram illustrating a "snap shot" of part of the Yangtze Platform during the Early Cambrian, and the various facies of the Xiannudong Formation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. We observed no evidence of archaeocyath-built reefs in the Xiannudong

Formation, either as in-situ reef boundstone or as debris in the float-rudstone. All

archaeocyaths in the float-rudstone appear as individual cups, with no indication that they

were eroded from a reef. Therefore, we conclude that, although archaeocyaths were

abundant in the northern part of the Yangtze Platform, they were not building reefs. The

float-rudstone is interpreted as storm-chumed archaeocyathan communities that were

living between the microbial reefs.

At both the Shatan and Fucheng, the dominating facies indicate a shallow-water,

high-energy environment on the carbonate platform. At Shatan, ooid shoals dominate the

lower two-thirds of the section, overlain by low-relief stromatolites and thrombolites,

followed by a 12-m unit of oncolite (Figure 2.3). This indicates that adjacent to the high-

energy ooid shoal facies was a low-energy environment that received occasional

moderate energy conditions. Shifts between the ooid shoals, microbial reefs, and oncoid

bearing micrite represent shifts between these different facies due to small sea level shifts

or storm events. Meanwhile, the Fucheng section contains numerous units of microbial

reefs with topographic relief and an inter-reefal community of archaeocyaths. Adjacent

to this reefal environment, were ooid shoals and possibly a deeper (deep subtidal)

environment or a restricted lagoon where silty/sandy lime muds were being deposited

(Figure 2.18).

To the south of Fucheng, the Xinchao deposits represent a lagoon with abundant

peloidal micrite, patchy microbial build-ups, and few burrows. Occasionally, storms or

strong tidal currents would cause shifts in the ooid shoals causing ooids to wash over into

the lagoon.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Implications for the Yangtze Platform

Overall, the paleoenvironmental scene indicated by the lithology and facies

associations is a carbonate platform. This conclusion is based on the presence of shallow

water carbonates deposited across the entire Yangtze Platform with offshore, slope and

basinal sediments known from the south and east of the platform (APC, 1985; Zhang and

Pratt, 1994; Zhu et al., 2001), and based on the associated carbonate facies. Furthermore,

similar carbonate facies and associations as presented here are documented from the

southern part of the Yangtze Platform in Yunnan Province by Debrenne and Jiang (1989).

We attribute the minor amounts of silt and frne-sand present in all facies to wind-blown

processes, which probably was derived from exposed strata in the northwest.

The lithofacies of the Xiannudong Formation indicate an equatorial or subtropical

(below 30° north or south) geographic setting for the Yangtze Platform in which

carbonate sediments, such as ooids, peloids, lime mud, and reefs are typically cultivated

(Wilson, 1975). This inferred position of the Yangtze Platform based on sediments

supports the paleogeographic reconstructions in which the position of the Yangtze

Platform was within 30° north or south of the equator, but it does not support the high

latitude paleogeographic reconstructions.

Acknowledgments

Funding for this project was received from the Geological Society of America,

American Museum of Natural History, the Institute for Cambrian Studies, the

Paleontological Society, and the Arizona-Nevada Academy of Sciences. Reviewers at

UNLV added to this manuscript with constructive comments.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTERS

CHEMOSTRATIGRAPHIC ANALYSIS OF EARLY CAMBRIAN MICROBIAL

REEFS, ARCHAEOCYATHAN REEFS, AND ASSOCIATED FACIES IN NEVADA

AND SOUTH CHINA: A SURPRISING STORY OF STASIS

Abstract

Mid-to late Early Cambrian sections in Nevada and South China were analyzed

for ô^^C for correlation purposes and to better describe the chemical these stratigraphie

sections represent. Data reveal stasis in all the Ô^^C records, which strongly contrasts

with the Early Cambrian ô^^C isotopic profile for the Siberian Platform. We hypothesize

three possible explanations for the stasis in the Ô^^C isotopes: (1) diagenetic alteration, (2)

the sections represent only a brief interval of time, and (3) environmental control.

In the mid-Early Cambrian Xiannudong Formation in northern Sichuan and

southern Shaanxi provinces, microbial reefs are present in great abundance with

archaeocyaths occurring either as low abundance dwellers or as transported debris

between the microbial reefs. However, the mid-Early Cambrian Poleta Formation in

Nevada contains archaeocyathan-microbial reefs in which archaeocyath abundance

increases upsection. Neither of the static ô‘^C signatures for the Poleta, centered near

07oo, and the Xiannudong, approximately -2.0 to 0°/oo, are illustrated on the composite

ô^^C record for the Siberian Platform.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The late-Early Cambrian Tianheban Formation in Hubei province and the

Harkless Formation in Nevada represent some of the youngest archaeocyathan reefs in

the world. For the Tianheban Formation, the Ô^^C values are positive, approximately 0 to

1,07oo, and appear to correlate with positive excursion X on the Siberian Platform Ô^^C

record. The Tianheban also contains well-developed archaeocyathan-microbial reefs.

The Ô^^C values from the Harkless Formation become increasingly positive upsection,

indicating that productivity was increasing.

None of the three hypotheses for stasis could be definitively accepted or rejected

but environmental control does appear to be the simplest and least problematic

explanation. Therefore, the static Ô^^C values are interpreted to represent interbasinal

environmental conditions. This suggests that interbasinal controls on Ô^^C may

commonly overprint the secular record.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction

The chemostratigraphy that we describe and interpret in this paper is part of a

larger study of the global collapse of archaeocyathan-calcimicrobial ecosystems toward

the end of the Early Cambrian using data from the Yangtze Platform and Laurentia. The

study involves two archaeocyath-rich intervals in Nevada, U.S.A. (Poleta Formation and

Harkless Formation) and two approximately coeval intervals in northern South China

(Xiannudong Formation and Tianheban Formation) (Figs. 3.1, 3.2).

Because of endemic faunas, global biostratigraphic correlation of Lower

Cambrian strata is problematic. It is commonly hypothesized that systematic variations

in ô^^C, as recorded in carbonate rocks, are secular, and such variations are used by many

researchers as correlation tools and as proxies for paleoclimatic, paleo-ocean chemical,

and paleotectonic change (Brasier 1993; Corsetti 1998; Veizer et al. 1999; Montanez et

al. 2000). To date, the Siberian Platform is the only continent for which a composite

Ô^^C profile has been constructed for the Early Cambrian. The Siberian Platform profile

reflects a very dynamic pattern of positive and negative excursions for the Ediacaran

through the Early Cambrian (Brasier et al. 1994; Brasier and Sukhov 1998) (Fig. 3.3).

In an attempt to use carbon isotopic chemostratigraphy as a tool for correlating

Laurentian strata with Yangtze Platform strata, and also to test the usefulness of the

Siberian Platform composite profile for global correlation, we conducted a meter-scale

chemostratigraphic study of the four archaeocyath-rich formations mentioned above. The

results indicate surprising intervals of isotopic stasis. Although this isotopic study turned

out to be disappointingly unhelpful for intercontinental correlation, we report important

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NV266 Harke tewart's \ section , Mill '^NV774 Mount Gold Point* AfvDun^

10 km North

Figure 3.1. A, Map of China showing the location of the three measured sections (stars). B, Location map of Esmeralda County, Nevada showing the locations of the with Poleta Formation (Stewart's Mill locality) (star) and the Harkless Formation (triangle). Shaded pattern indicates major mountain ranges in Esmeralda County.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Archaeocyatha s (°/oo PDB) Percent Extinction Middle -5 -4 -3 -2 -1 0 1 2 20 60 100 _l I I L Cambrian •

C < c I .o Botomian- IX Toyonlan mc Crisis c

E c (0 .s O E I m ^ J ÿ v i l l VII

VI çg (0 "c LJJ Ëca "O Archaeocyathan ' — x ' v f / genera

J ll „ II ■ — ' 1

9 z

Ediacaran

-5 -4 -3 -2 “1 1 2 3 0 80 160 No. of archaeocyathan genera Figure 3.3 Composite profile for the Siberian Platform. This composite contains the most data for any Early Cambrian locality in the world. No Early Cambrian composite sections for the U.S or China exist with such large data sets. Roman numerials designate positive isotopic excursions. N-D is the Nemakit-Daldynian (From Brasier et al. 1994).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. new isotopic data for stratigraphie intervals of Laurentia and the Yangtze Platform, and

raise questions about the global uniformity of values in the Early Cambrian.

Methods

We measured and sampled five sections for isotopic analyses, three sections in

South China and two sections in the Great Basin. The three sections in South China

include two sections (Fucheng and Xinchao) of the Xiannudong Formation and one

section of the Tianheban Formation. The two Great Basin sections are the Stewart’s Mill

locality of the Lower Member of the Poleta Formation and a section of the upper part of

the Harkless Formation (Site 1 of Hicks, 2001) both of which are in Esmeralda County,

Nevada, U.S.A (Fig. 3.1).

We sampled each section at approximately 1-meter intervals, except where the

strata were covered or visibly unsuitable for isotopic analyses, such as areas with

numerous veins or obvious erosional features. All collected samples were thin sectioned

and petrographically analyzed for obvious diagenetic alteration. The billet cut from the

slides was retained for drilling. Each slide is the mirror image of the billet from which it

was cut, therefore the billet can be drilled in the same location as described in thin

section. We analyzed each thin section by cathodoluminescence (CL) at UNLV, under

10-11 kV with a 0.8-0.9-milliamp beam current. Areas with dark red luminescence

represent unaltered regions, while areas with bright orange and red luminescence have

been diagentically altered (Corsetti, 1998). Dark luminescence or non-luminescence

characterizes carbonate rocks that have not been dissolved and replaced by Mn-bearing

meteoric water (Corsetti, 1998). Only the best-preserved samples were chosen for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. isotopic analysis; o f286 samples collected for isotopic analyses, 194 proved to be

suitable. At UNLV, we microdrilled approximately lOOpg of powder from each suitable

sample. The powdered samples were analyzed for and 6^*0 on the University of

Maryland’s Isoprime Dual Source Gas Source Mass Spectrometer under the direction of

Alan Jay Kaufman. The powdered samples were treated with 100% pure phosphoric acid

in order to convert the carbon to CO 2 gas. The gas was then analyzed for Ô^^C. All

carbon and oxygen samples were corrected to a house standard that is corrected to PDB

and are measured in parts per mil (7oo).

All percentages of archaeocyaths and other constituents of the carbonates were

calculated from point counts on a Hacker Instrument, James Swift point counter. For

each 3 x 2 inch slide, 400 points were counted at a four-increment spacing.

Stratigraphy and Sedimentology

Xiannudong Formation

In the two measured sections of the Xiannudong Formation, we distinguish three

lithofacies: microhial reefs, silty/sandy micrite, and oolite. The thickness of the

Xiannudong Formation differs greatly between the two sections. At the Fucheng

section, located near the town of Fucheng in southern Shaanxi Province (Fig. 3.1), the

67.3 meter thick Xiannudong Formation conformably overlies the Guojiaba Formation

(Fig. 3.4). At this section, the Xiannudong Formation contains numerous cyclical

microbial reef complexes with archaeocyathan-rich float-rudstone between the

stromatolite and thrombolite reefs. Archaeocyaths did not form reefs in any of the

studied Xiannudong Formation localities. Instead, archaeocyaths form an inter-reef

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD Q.O C Q.g

■D CD

C/) Fucheng section Xincho section W Xiannudong Formation Stewart's Miii section o ' Xiannudong Formation Top of Yanwanbian Poieta Formation 3 Top of Fm. 0 -I'S -I'e -l'4 -1*2 -Ï0 -b.' b ' 2 e x p o s u re 3 ‘ 107- CD 8 100-

(O' 3" 90 1 3 CD 80

3.C 3" 10 \ CD

CD ■D 60 Q.O C I a o 50-I 3 ■D O Guojiaba ,— i— r pm -12 -10 -8 -2 ’ o""' 2 40 j 5180 513C 40, Q.CD Key to stratigraphie columns 8180 513C 30 J 0 # Oolite Silty/sandy micrite

■D 2 0 -I CD BH Microbial reefs Archaeocyath float-rudstone C/) C/) ^ 1 Oncoiite 10 Peloidal wackestone/packstone Gralnstone 1 1 Covered -lb -lb -i'4 -i'2 -I'o -b 6 ' i ° Campito S l 8 n s T 3 r Guojiaba 5l3c Fm, Fm. Figure 3.4. Stratigraphie columns from the two sections of the Xiannudong Formation of China and the one section of Poieta from Nevada, U.S.A., with the respective S 13c and 51*0 values. Stratigraphie units are in meters. Ô13C and 5180 values are in parts per million (°/oo) PDB. community and are present in very low abundance (approximately 1.0%) between the

microbial reefs. The archaeocyathan floatstone/rudstone is enriched in ‘regular’

archaeocyaths (Orders Ajacicyathida and Capsulocyathida)', this contrasts with the

archaeocyaths present in the microbial reefs themselves, which are all ‘irregulars’ (Order

Archaeocythidd). Interbedded and occasionally interfingering with the microbial reef

complexes is cross-bedded oolite.

Multiple units of wavy-bedded, moderately bioturbated, silty/sandy micrite repeat

throughout the Fucheng section. We interpret these reoccurring changes in lithofacies to

represent small sea-level fluctuations and/or small environmental changes, which shifted

the adjacent facies (Hicks and Rowland, in prep).

The Xinchao section of the Xiarmudong Formation is located approximately 15

km to the west of the Fucheng section, near the town of Xinchao in northern Sichuan

Province (Fig. 3.1). Here, the Xiannudong Formation is 114 m thick, which is much

thicker than in the Fucheng section. Overall, the Xiannudong in the Xinchao section

consists of multiple, thick intervals of thinly bedded micrite to pelloidal, silty/sandy

micrite and oolite. Only two intervals contain archaeocyaths, which occur as constituents

in a float-rudstone that is similar to the Fucheng float-rudstone. The multiple cycles of

stromatolitic and thrombolitic reefs with micrite and silty/sandy micrite and oolite, which

characterize the Xiannudong Formation at Fucheng, do not occur at Xinchao. The

presence of thick intervals of peloidal micrite indicates a low energy setting for the

Xinchao section. Shifts in the facies here are also attributed to possible sea-level changes

or other environmental changes (Hicks and Rowland, in prep.)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lower Member of the Poieta Formation

The Lower Member of the Poieta Formation is exposed in Esmeralda County,

Nevada and in the White-Inyo Mountains in California. The section used in this study is

the Stewart’s Mill locality, which lies northwest of the town of Goldpoint, Nevada (Fig.

3 .1). At the Stewart’s Mill locality, the Poieta Formation conformably overlies the

Campito Formation. The Lower Member of the Poieta at Stewart’s Mill is approximately

107 meters thick (Rowland and Gangloff 1988). Similar to the Xiannudong at Fucheng,

the Lower Poieta illustrates laterally adjacent facies changes with a delicate interplay

between archaeocyathan-microbial reefs and migrating oolitic shoals. Repeating

intervals of archaeocyathan-microbial reefs, packstone, wackestone, ooid grainstone, and

grapestone range between 1 to 10 meters thick.

The reefs range from thrombolite to i?ewa/«5-archaeocyathan boundstone, with

sparse to abundant archaeocyaths, trilobite fragments, and echinoderm plates. These

thrombolitic to i?cwa/cw-archaeocyathan boundstones have been interpreted to represent a

shallow subtidal depositional environment (Rowland and Gangloff, 1988). Laterally

associated wackestone and micrite represent material shed from the boundstone.

Upsection the boundstone becomes more cavernous, with archaeocyaths

becoming more abundant (Rowland and Gangloff 1988; Rowland and Hicks 2004). The

boundstones vary from 2 m to approximately 20 meters thick with conspicuous in-situ

‘regular’ and ‘irregular’ type archaeocyaths, Renalcis, md Epiphyton. Continuing

upsection, archaeocyaths increase in abundance to a maximum of 38% by volume

(Rowland and Gangloff 1988). Archaeocyaths reported from the Stewart’s Mill locality

include Protopharetra, Dailycyathus, Retilamina, Diplocyathellus, Ethmophyllum, and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. other unidentifiable archaeocyath genera (Rowland and Gangloff 1988). The diversity

decreases upsection, with Protopharetra and Dailycyathus dominating the upper

boundstones.

Interbedded with the boundstone are oolite, packstone, and fossiliferous

wackestone (Rowland and Gangloff 1988). These various lithologies represent adjacent

facies that replaced one another as sea-level rose, shifting boundstone and oolite facies

shoreward.

Tianheban Formation

The Wangjiaping section of the Tianheban Formation is located near the city of

Yichang, Hubei Province, China (Fig. 3 .1). The Tianheban Formation in this section is

69 meters thick (Fig. 3.5). The base of the Tianheban Formation contains thin interbeds

of siltstone and micrite, with micrite becoming more dominant upsection. Overlying the

micrite are oolite and fossiliferous oncoiite. Burrow-mottled micrite dominates the

middle portion of the section with more fossiliferous oncoiite overlying the mottled

micrite. Above this oncoiite lies the single recognizable, mound-shaped, archaeocyathan

reef within this section (2.1 m thick and approximately 1.8 min length). This reef

contains only one genus of archaeocyath, Archaeocyathus sp., which comprises over 33%

of the reef by volume. Immediately overlying the archaeocyathan-microbial reef is thin

veneer of fossiliferous-ooid-oncoid-packstone. The remaining section contains massive,

burrow-mottled micrite, with one interval of nodular micrite (Fig. 3.5).

In contrast to the archaeocyath-rich sections of the Xiarmudong, Poieta, and

Harkless formations, the Wangjiaping section records a low-energy environment. In a

previous study of the Tianheban Formation at the Wangjiaping section, Debrenne et al.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V\fengjiaping section Tianheban Formation jshiiongdong Hafkless Formation I—I—I— f -10 -9 -8 -7 -2 Top of exposure

upper Harkless section

1-14 -10 -4 0

0"° I -14 -10 -4 0 5180°, 513c°/_ # 0

20 -

401

10-

3 0 1 1 mm

Base of 201 exposure 5 1 8 0 S13C ___ ## 0 K Oolite 0 0 W Oncolite/packstone i t Peloidal wackestone/ lo i i t n covered Pookstone

□ □ 1 1 Archaeocyath-calcimicrobial A m Micrite

I— I— r ~T Calcareous shale -10 -9 -8 T . r 0 Shipai m i Sandstone M3c°/_ Fm. Packstone

Packstone conglomerate Figure 3.5. Measured sections and corresponding ôl^O and Ôl3C chemostratigraphy for the Waijiaping section of the Tianheban Formation, and the upper part of the Harkless Formation. Stratigraphie units are in meters. ôl3c and ôl80 values are in parts per million (o/oo) PDB.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (1991) intrepreted the archaeocyathan-microbial reef to have formed below wave base in

low-energy conditions. That interpretation was based on the presence of abundant lime

mud and a lack of fibrous cements, which are known to form in high-energy

environments (Mathews 1974). However, samples collected by us do contain fibrous

cements along with blocky cements in a fenestral fabric, in particular birds-eye fabric.

The reef flanks tend to have fibrous cements, while the reef base and core contain

fenestral birds-eye fabric. The reef crest contains both fibrous cements and birds-eye

fabric. We interpret these variations in cement to be due to the reefs exposure to

differential wave energy. The reef flank and crest would have been in a slightly higher

energy regime than the base or core, therefore facilitating the growth of primary fibrous

cements.

We agree with Debrenne et al. (1991) that this reef formed in a low-energy

environment, but the distribution of cements leads us to conclude that the reef formed in

very shallow water, shallower than below wave base. Fenestral fabrics are interpreted as

a shallow water desiccation fabric, such as would occur on a supratidal flat (Wilson,

1975). Tides or storms would have increased the energy around the reef, resulting in the

formation of fibrous cements on the flanks and crest. This would also explain the

fossiliferous ooid, oncoid packstone that flanks and overlies the reef.

Harkless Formation

The Harkless Formation section analyzed in this study is located in Lida Valley

northeast of the town of Goldpoint, Esmeralda County, Nevada (Fig. 3.1). This section

of the Harkless Formation contains three patch reefs (Hicks, 2001). The basal portion of

the section consists entirely of highly weathered, green/tan shale with micaceous partings

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Fig. 3.5). The shale is slightly bioturbated and contains the ichnofossil Plcmolites.

Abruptly overlying the shale is a single bed of highly fossiliferous, green, oncolite-

bearing wacke-packstone. Fossils in this bed include trilobite spines, echinoderm plates,

oncoids with nuclei of trilobite spines, and highly broken, unidentifiable fossil fragments.

Conformably overlying the oncoiite wacke-packstone bed is rubbly, green/tan shale with

micaceous partings. This shale has the same characteristics as the shale below the oncoid

wacke-packstone.

A cliff-forming, four-meter-thick interval of highly fossiliferous, gray packstone

overlies the rubbly shale. Within this massive packstone unit, lie three 1.5 to 2.0-meter-

thick archaeocyathan-calcimicrobial reefs. All three reefs are within the same horizon in

outcrop and are separated from each other by approximately 12 meters of packstone. The

contact beneath each reef is sharp and conformable, while the overlying and flanking

contacts are erosional. A high diversity of fossils and coated grains is present within the

packstone, including Salterella, echinoderm plates, trilobite spines, brachiopods, oncoids,

ooids, and peloids. Overlying the gray packstone is a wavy bedded packstone

conglomerate in which the packstone clasts are identical to the underlying packstone.

The packstone clasts lie within a calcareous sandstone matrix; no preferred orientation of

the clasts is apparent, but imbricated clasts do occur at the contact with the overlying

sandstone.

The packstone conglomerate is gradationally overlain by orange, coarse- to fine

grained, calcareous sandstone. The calcareous sandstone is wavy bedded with imbricated

packstone conglomerate clasts in the basal portion. These imbricated clasts indicate a

paleocurrent direction toward paleo-north (offshore). Faint ripple marks occur in the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. upper portion of the calcareous sandstone. In some places, the overlying calcareous

sandstone is in direct contact with the reef, with an erosional boundary between the two.

Above the calcareous sandstone are interbedded green/tan rubbly-weathering shale,

siltstone and massive bedded quartzitic sandstone.

The presence of primary cavities filled with fibrous cements and stratified lime

mud throughout the fiamestone indicates that these reefs formed in a shallow subtidal

zone with moderate to high-energy wave activity (Hicks, 2001).

Biostratigraphy

Biostratigraphy did not prove to be very usefiil for correlating the Nevada sections

with the Chinese sections. The approximate correlation between the Poieta and

Xiannudong formations is based on trilobites and their correlation to the Siberian

Platform (Figure 3.2). The Lower Member of the Poieta Formation lies within the

Nevadella Zone, upper Montezuman Stage (Laurentian Stage), which correlates to the

upper Atdabanian to lower Botomian stage of the Siberia Platform (Zhuravlev and Riding

2001; Hollingsworth 2004; Shergold and Cooper 2004) (Fig 3.2).

The age of the Xiannudong Formation is slightly less well established. Yuan and

Zhang (1981), Qin and Yuan (1984), and Zhang (1989) all place the Xiannudong

Formation as straddling several Chinese trilobite biozones, Malungia and Yunrtanaspis-

Yiliangella, which are in the Tsanglangpu (Canglangpuan) Stage of China. However, Ye

et al. (1997), and Yuan et al. (2001) place the base of the Xiannudong Formation a bit

lower, in the upper Eoredlichia-Wutingaspis Zone, but still in the Malungia Zone, and the

Yuannanaspis-Yiliangella Zone, straddling the upper Qiongzhusi (Chiungchussan) Stage

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and the lower Tsanglangpu (Canglangpuan) Stage. This latter placement correlates to the

upper Atdabanian to lower Botomian on the Siberian Platform, and upper Montezuman

Stage on Laurentia (Zhuravlev and Riding 2001; Hollingsworth 2004; Shergold and

Cooper 2004) (Fig. 3.2). These age determinations for the Xiannudong are based on the

presence of the trilobites Ma/wngza gramilose, Micangshania gracilis (Yuan and Zhang

1981; Zhang 1989),Yunnanocephalus, Zhenbaspis, Eoredlichia, and Kuanyangia (Yuan

et al. 2001). Thus the Lower Poieta and the Xiannudong formations are approximately

the same age, but, due to endemism of Laurentian and Yangtze Platform trilobites, their

precise relative ages are not known.

The Tianheban Formation lies within in the Megapalaeolenus Zone in the upper

Tsanglangpu (Canglangpuan) Stage (Yuan and Zhang 1981; Zhang 1989; Ye et al. 1997;

Yuan et al. 2001). This age is based on the presence of the trilobites Megapalaeolenus

deprati, Xilingxia ichangensis, Kootenia, and Redlichia dobayashii (Yuan and Zhang

1981). The Megapalaeolenus Zone correlates to the upper Toyonian of the Siberian

Platform and upper Bonnia-Olenellus trilobite zone, Dyeran Stage of Laurentia

(Zhuravlev and Riding 2001; Hollingsworth 2004).

The Harkless Formation lies within the Bonnia-Olenellus Zone, Dyeran Stage of

Laurentia (Hollingsworth, 2004). The section analyzed in this study includes just the

upper part of the Harkless Formation (the Saline Valley equivalent strata), which is upper

Bonnia-Olenellus Zone; this correlates to the upper Toyonian Stage of the Siberian

Platform. The age of the upper Harkless is constrained by the trilobite Wanneria, which

is medial Bonnia-Olenellus (Palmer and James, 1979). This age is further supported by

the archaeocyath assemblage within the upper Harkless Formation, which is similar to the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. archaeocyathan assemblage of the well studied, medial to uppev-Bonnia-Olenellus zone,

Forteau Formation of Labrador (Palmer and James 1979; James and Klappa 1983; Hicks,

2001; Hollingsworth 2004; Palmer, personal communication).

Chemostratigraphic Analysis

Results and Discussion

The and 6^*0 values for each section are provided in Tables 3.1-3.5 of

Appendix 1. Figures 3.4 and 3.5 graphically illustrate the isotopic values with their

corresponding measured sections. The most obvious feature of each of the plots is

the lack of large excursions. This is surprising, in view of the pattern on the Siberian

Platform (Fig. 3.3). Four of the plots show relative stasis. The extreme example is the

Lower Member of the Poieta Formation; though more than one hundred meters of

section, the data record only a very gradual drift of approximately l7oo (Fig. 3.4).

The Chinese sections display a slightly unstable Ô^^C record, but none of them show

excursions comparable to those on the Siberian Platform. The ô^^C values of the

Harkless Formation vary from 0 to -3.87oo, showing the largest variation of the five

intervals studied. However, the Harkless data set contains only four points (Fig. 3.5), so

it is not possible to draw any meaningful conclusions about the isotopic trends within the

Harkless Formation or to use these values to correlate the Harkless to the Siberian

Platform.

There are three possible explanations for the surprising stability of the 0*^C values

obtained in this study; (1) diagenetic alteration has obliterated the primary isotopic

signature, (2) the sections studied represent only very brief intervals of Early Cambrian

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. time, or (3) local environmental factors caused the values recorded in these localities

to be different than those recorded on the Siberian Platform. Implicit in the second

hypothesis is the conclusion that local environmental factors can overprint the secular

values, and therefore the Siberian Platform profile cannot be used as a tool for

global correlation. We will consider each of these three hypotheses.

Diagenetic Alteration Hypothesis

We consider diagenesis to not be the controlling factor behind all of the invariant

data values. As mentioned in the “Methods Section”, both field and laboratory

precautions were taken to avoid altered samples. We further checked for alteration by

analyzing the 5^*0 in each sample. During carbon isotopic analyses, 6^*0 was also

measured and plotted (Figs 3.4 and 3.5). Oxygen isotopic values are more vulnerable to

diagenetic alteration than are carbon isotopic values (Kennedy, 1996). For carbonates,

the primary signature has a high preservation potential compared to 5^*0 because

carbonate is a major component of carbonate rocks but only a minor component in

digenetic fluids (Kennedy, 1996).

Large negative excursions in the 5^*0 values, such as the -187oo value observed at

approximately 10 m in the Poieta section (Fig. 3.4), accompanied by a smaller 1.07oo

negative jog suggests alteration. The strongly negative nature of the 5^*0 values for

the Poieta and Harkless formations suggests that strata were exposed to fluids, but this

does not necessarily mean that the carbon isotopic values, which are much more robust

than oxygen, were altered. Most Proterozoic and Cambrian sections of the southwestern

U.S. show 5^*0 values more negative than -107oo (ppm), and the values are still

considered primary (Brasier 1993; Corsetti and Kaufman 1994).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. No single test exists that will unequivocally determine whether a particular

sample is altered or unaltered. Samples that pass the field, pétrographie, and

cathodoluminescence tests are assumed to provide primary signatures (Corsetti and

Kaufman 1994; Corsetti 1998). Consequently, most of the Poieta Formation Ô^^C values

are considered to be primary, however all three Chinese ô^^C plots are viewed with some

caution because Ô^^C and 5^*0 covary. Because two of the four Harkless Formation

isotopic samples were obtained fi'om highly fossiliferous packstone, and contamination

due to vital effects from organisms is possible, we do not have confidence that the

Harkless Formation ô^^C values are very meaningful.

Brief Interval Hvpothesis

Because the absolute values of the ô^^C data from Nevada and China are similar

to coeval sections in Siberia (Fig. 3.3) (Brasier et al. 1994; Brasier and Sukhov 1998) and

Mongolia (Brasier et al. 1996), each of the stratigraphie intervals examined in this study

may represent only a brief “snapshot” of Early Cambrian time that is too brief to capture

the Ô^^C excursions that have been documented on the Siberian Platform. The composite

ô^^C curve for the Siberian Platform integrated multiple formations that range from 110

to 250 m in thickness and span from the Ediacaran to the base of the Middle Cambrian

(Brasier et al. 1994). The Poieta Formation and the Xinchao section of the Xiannudong

Formation are over 100 m thick, but the exact length of time represented in each of these

formations cannot be established.

In order to make any real conclusion on the importance of the stasis in the Poieta

Formation, we need to sample and analyze more sections of the Poieta Formation and

also attain better age constraints and subsidence rates for backstripping analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Environmental Control Hypothesis

With respect to the environment that would produce such invariant Ô^^C values,

we have several hypotheses. Positive ô^^C values could indicate that primary

productivity is high and photosynthesizing organisms are preferentially sequestering the

light carbon isotope, ^^C and/or increased stratification in the oceans with organic

preservation in anoxic waters (Corsetti 1998; Kump and Arthur, 1999; Montanez et al.

2000). Conversely, negative ô^^C values typically reflect environments in which organic

carbon is exhumed or primary productivity is greatly diminished (Corsetti 1998;

Montanez et al. 2000). These variations in ô^^C are interpreted as global and are used as

a global correlative tool for the Neoproterozoic and Cambrian strata (Brasier 1993;

Corsetti 1998; Veizer et al. 1999; Montanez et al. 2000). However, the Ô^^C record may

also be intrabasinal, where local values can mask or smooth primary signals (Pelechaty et

al. 1996).

The 0 to -2.07oo ô^^C values recorded in the Xiarmudong Formation indicate an

environment in which the primary productivity appears to be slightly diminished. The

presence of both thrombolitic and stromatolitic reefs argues against low primary

productivity as these types of reefs are hypothesized to have been built by cyanobacteria

(Riding 1990). Therefore, low productivity does not explain the negative ô^^C values

recorded in both sections of the Xiarmudong Formation.

With Ô^^C values around Oto -l7oo, the Poieta Formation records a similar Ô^^C

record as the Xiannudong Formation. However, in the Poieta Formation, archaeocyaths

are the main reef-builder with an upward trend toward increasing archaeocyath

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. abundance from 3% to 38% (Rowland and Gangloff 1988). This presents an interesting

contrast with the Xiannudong Formation, where archaeocyathan reefs do not occur.

The values of the Tianheban Formation are also static in comparison with

the Siberian Platform, but slightly positive (0 to l7oo). The small positive excursion at

the base of the section may represent excursion X on the Siberian Platform plot (Fig.

3.3). Based on the positive signature, the Tianheban best correlates to the

uppermost Early Cambrian, which correlates well with the biostratigraphic control. The

data presented in this study illustrates that much more work needs to be done on isotopic

signals globally for this interesting period of the Early Cambrian.

Conclusions

The following general conclusions were made based on this study.

1) None of the five chemostratigraphic sections [Poieta, Xannudong (Fucheng and

Xnchao), Tianheban, and Harkless formations] show any distinct 5^^C

excursions; rather they record a history of isotopic stasis. This result is not useful

for intercontinental correlation.

2) We conclude that the stasis in the ô^^C signatures, especially for the Poieta

Formation, is not entirely due to diagenetic alteration.

3) Because the Siberian Platform ô^^C record does not record the stasis observed in

the Poieta Formation, the Lower Poieta S^^C results either (1) record a localized,

basinal condition, or (2) the Siberian Platform profile does not record a globally

correlatable ô^^C signature.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4) Similar to the Poieta Formation, the record of the Xannudong is also

interpreted to record an intrabasinal environmental condition.

5) The positive values of the Tianheban Formation correlate well with the mid-to

latest Early Cambrian (Toyonian) on the Siberian Platform at Brasier et al.’s

(1994) positive excursion X. However, interpretations are viewed with caution

due to the covariance observed between and 6^*0.

6) Each of the sections analyzed for produced a static record that is not

present within the record of the Siberian Platform. We suggest that

interbasinal environmental controls may have a more profound effect on the

secular record than previously imagined.

7) With the existing data, none of the three hypotheses for the surprising isotopic

stasis observed in this study can be falsified, although the localized environmental

hypothesis is accepted as the least problematic.

8) More chemostratigraphic data need to be collected from all these formations,

in particular the Poieta and Harkless formations, in order to better understand the

record in Laurentia and globally.

Acknowledgments

Financial assistance for this study was provided by the Institute for Cambrian

Studies, the American Museum of Natural History, and the Geological Society of

America. Interpretations for this study were greatly helped by A. J. Kaufman and G.

Jiang.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. References

Brasier, M.D., 1993, Towards a carbon isotope stratigraphy of the Cambrian System:

potential of the Great Basin succession, in Hailwood, E.A., and Kidd, RB , eds..

High Resolution Stratigraphy: Geological Society Special Paper, v. 70,

p. 341-350.

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remarks on some Lower Cambrian sediments for the Yangtze platform (China):

Bulletin of the Society Geology of France, v. 163, p. 575-583.

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chemostratigraphy for intrabasinal correlation: Vendian strata of northeast

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J.G., and Smith, A G , eds., A Geologic Time Scale 2004: Cambridge University

Press, Cambridge, p. 147.

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northern margin of the Sichuan Basin: Proceedings of the 30* International

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and southwestern China: Geological Society of America Special Paper 187,

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horizons in the Lower Cambrian Fucheng section, south Shaanxi Province:

implications for regional correlations and archaeocyathan evolution: Acta

Palaeontologica Sinica, v. 40, p. 115-129.

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archaeocyathids in China: Journal of Southeast Asian Sciences, v. 3, p. 199-124.

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R , eds.. The Ecology of the Cambrian Radiation: Columbia University Press,

New York, p. 1-7.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.1. ôand values for the Poieta Formation. Sample numbers correspond with stratigraphie measurements (meters). Corr = corrected data, sd = standard deviation, PDB = Pee-dee beleminite standard. sample raw corr sdA^C ^^Oraw corr sd PDB PDB PDBPDB

P-106 1.077 -0.758 0.004 -21.24 -14.64 0.009 P-I05 0.816 -1.019 0.004 -21.859 -15.259 0.009 P-104 0.963 -0.872 0.005 -23.573 -16.973 0.016 P-103 0.915 -0.92 0.004 -22.843 -16.243 0.007 P-102 1.23 -0.605 0.003 -21.851 -15.251 0.007 P-101 0.894 -0.941 0.006 -24.14 -17.54 0.007 P-IOO 0.733 -1.102 0.006 -20.227 -13.627 0.011 P-99 0.906 -0.929 0.005 -19.681 -13.081 0.011 P-98 -0.25 -1.106 0.006 -12.395 -12.597 0.006 P-96 0.804 -1.031 0.004 -24.037 -17.437 0.011 P-94 0.934 -0.901 0.004 -19.406 -12.806 0.011 P-93 1.164 -0.671 0.005 -21.089 -14.489 0.008 P-92 1.07 -0.765 0.006 -20.319 -13.719 0.006 P-90 0.934 -0.901 0.004 -23.88 -17.28 0.006 P-89 1.118 -0.717 0.006 -18.533 -11.933 0.005 P-88 1.24 -0.595 0.005 -21.393 -14.793 0.007 P-87 1.092 -0.743 0.006 -18.21 -11.61 0.011 P-86 1.253 -0.582 0.006 -18.303 -11.703 0.015 P-85 1.154 -0.681 0.005 -18.511 -11.911 0.008 P-83 1.306 -0.529 0.004 -17.46 -10.86 0.013 P-82 1.311 -0.524 0.002 -20.404 -13.804 0.008 P-76 1.065 -0.77 0.006 -20.678 -14.078 0.006 P-74 1.277 -0.558 0.004 -19.46 -12.86 0.011 P-73 0.963 -0.872 0.005 -20.912 -14.312 0.011 P-72 1.125 -0.71 0.004 -19.559 -12.959 0.01 P-71 0.958 -0.877 0.004 -20.73 -14.13 0.009 P-70 1.161 -0.674 0.002 -19.641 -13.041 0.004 P-68 1.32 -0.515 0.006 -18.907 -12.307 0.012 P-67 1.184 -0.651 0.002 -18.954 -12.354 0.009 P-65 1.501 -0.334 0.004 -19.968 -13.368 0.007 P-64 1.512 -0.323 0.006 -19.098 -12.498 0.009 P-62 1.497 -0.342 0.002 -19.865 -13.206 0.005 P-6I 1.441 -0.398 0.004 -19.732 -13.073 0.01 P-60 1.268 -0.571 0.004 -21.421 -14.762 0.013 P-58 1.784 -0.055 0.006 -18.531 -11.872 0.01 P-57b 1.629 -0.21 0.003 -21.266 -14.607 0.005 P-57 1.648 -0.191 0.004 -19.919 -13.26 0.008 P-56 1.492 -0.347 0.003 -19.623 -12.964 0.005 P-55 1.324 -0.515 0.003 -21.405 -14.746 0.005 P-53 1.572 -0.267 0.003 -21.397 -14.738 0.005 P-52 1.825 -0.014 0.002 -21.44 -14.781 0.014 P-50 1.401 -0.496 0.003 -21.39 -14.965 0.005 P-49 1.441 -0.456 0.004 -20.452 -14.027 0.007 P-43 1.455 -0.442 0.006 -20.497 -14.072 0.006 P-42 1.476 -0.421 0.003 -20.828 -14.403 0.006 P-41 1.291 -0.606 0.005 -22.783 -16.358 0.01 P-40 1.45 -0.447 0.004 -20.421 -13.996 0.003 P-39 1.256 -0.641 0.002 -19.573 -13.148 0.008 P-35 1.338 -0.559 0.006 -19.67 -13.245 0.02

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.1. Continued Ô^^C and values for the Poieta Formation.

sample l^Ctaw corr s d % 1*0 raw 1*0 corr sd 1*0 PDBPDB PDBPDB

P-34 0.993 -0.904 0.004 -20.59 -14.165 0.009 P-33 1.239 -0.658 0.003 -20.279 -13.854 0.008 P-32 1.707 -0.19 0.002 -19.078 -12.668 0.007 P-30 1.684 -0.213 0.004 -19.997 -13.587 0.008 P-28 2.111 0.214 0.002 -17.862 -11.452 0.004 P-27 1.82 -0.077 0.002 -18.117 -11.707 0.004 P-23 1.943 0.046 0.004 -18.945 -12.535 0.002 P-21 1.913 0.016 0.003 -19.52 -13.11 0.006 P-20 2.302 0.405 0.004 -18.552 -12.142 0.009 P-18 1.107 0.251 0.002 -11.702 -11.904 0.003 P-17 2.82 0.923 0.002 -17.247 -10.837 0.011 P-13 2.498 0.601 0.003 -17.7 -11.29 0.005 P-11 1.376 0.52 0.004 -11.258 -11.46 0.005 P-11 0.53 -0.326 0.002 -17.59 -17.792 0.005 P-9 1.293 0.437 0.006 -11.418 -11.62 0.004 P-8 1.151 0.295 0.003 -11.408 -11.61 0.003 P-7 1.225 0.369 0.002 -11.561 -11.763 0.002 P-6 0.899 0.043 0.006 -12.161 -12.363 0.01 P-5 0.922 0.066 0.005 -12.281 -12.483 0.006 P-4 0.685 -0.171 0.004 -11.933 -12.135 0.005 P-3 0.487 -0.369 0.003 -12.515 -12.717 0.007 P-2 0.657 -0.199 0.002 -14.754 -14.956 0.003

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.2. and values for the Fucheng Section of the Xannudong Formation. sample l^Craw l^Ccorr s d l3 c 1*0 raw 1*0 corr sd 1*0 PDB PDB PDBPDB

F-51 0.307 -0.466 0.002 -9.405 -9.559 0.007 F ^ 8 0.157 -0.596 0.003 -8.633 -8.798 0.003 F-43 0.199 -0.574 0.003 -9.92 -10.074 0.006 F-42 0.109 -0.747 0.006 -9.996 -10.198 0.017 F-41 0.428 -0.325 0.006 -9.904 -10.069 0.008 F-40 0.515 -0.238 0.002 -10.124 -10.289 0.009 F-39 0.713 -0.143 0.004 -9.832 -10.034 0.005 F-38 0.643 -0.213 0.002 -10.011 -10.213 0.004 F-37 0.525 -0.18 0.004 -9.988 -9.996 0.006 F-35 0.41 -0.295 0.004 -10.113 -10.121 0.005 F-34 0.596 -0.109 0.002 -9.827 -9.835 0.006 F-33 0.55 -0.223 0.004 -9.723 -9.877 0.003 F-24 0.241 -0.615 0.003 -10.121 -10.323 0.003 F-23 -0.285 -0.99 0.003 -10.764 -10.772 0.01 F-22 -0.179 -0.952 0.005 -10.235 -10.389 0.006 F-21 -0.022 -0.878 0.006 -10.026 -10.228 0.006 F-20 -0.085 -0.79 0.002 -10.517 -10.525 0.004 F-19 -0.412 -1.117 0.004 -10.419 -10.427 0.01 F-16 -0.712 -1.417 0.004 -10.278 -10.286 0.006 F-7 -0.176 -0.949 0.005 -11.129 -11.283 0.007 F-6 -0.408 -1.113 0.003 -10.932 -10.94 0.005 F-4 -0.397 -1.253 0.003 -10.502 -10.704 0.007 F-3 0.114 -0.742 0.001 -10.89 -11.092 0.006 F-1 -2.386 -3.242 0.001 -11.593 -11.795 0.003

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.3. and values for the Xnchao Section of the Xannudong Formation sample l^Craw l^Ccorr s d l3 c ^*Oraw corr sdl^O PDB PDB PDBPDB X-50 0.25 -0.455 0.003 -8.05 -8.058 0.01 X-49 -0.073 -0.826 0.003 -9.618 -9.783 0.006 X ^ 3 0.254 -0.451 0.004 -9.693 -9.701 0.008 X-42 -0.133 -0.838 0.004 -9.237 -9.245 0.004 X-40 0.06 -0.645 0.002 -9.37 -9.378 0.005 X-38 0.133 -0.572 0.002 -9.225 -9.233 0.004 X-37 0.369 -0.336 0.006 -8.709 -8.717 0.004 X-36 0.162 -0.543 0.004 -9.313 -9.321 0.009 X-32 0.056 -0.649 0.003 -9.324 -9.332 0.004 X-31 0 -0.705 0.002 -9.445 -9.453 0.003 X-29 0.304 -0.401 0.004 -9.006 -9.014 0.006 X-26 0.313 -0.440 0.003 -9.139 -9.304 0.006 X-24 0.501 -0.204 0.003 -8.796 -8.804 0.005 X-23 0.438 -0.267 0.002 -8.452 -8.46 0.007 X-22 0.158 -0.595 0.004 -9.697 -9.862 0.004 X-21 0.284 -0.469 0.003 -9.177 -9.342 0.007 X-16 -0.546 -1.299 0.005 -10.750 -10.915 0.005 X-15 0.39 -0.315 0.003 -8.826 -8.834 0.002 X-14 0.760 0.007 0.004 -7.516 -7.681 0.007 X-13 0.123 -0.582 0.002 -9.281 -9.289 0.005 X-11 0.23 -0.475 0.006 -9.062 -9.07 0.008 X-10 0.818 0.113 0.002 -6.765 -6.773 0.007 X-9 -0.16 -0.865 0.004 -9.718 -9.726 0.007 X-8 -0.559 -1.264 0.003 -9.629 -9.637 0.009 X-3 0.746 -0.007 0.003 -7.383 -7.548 0.011 X-1 -0.573 -1.278 0.014 -8.269 -8.277 0.011 X-1 0.066 -0.639 0.003 -9.99 -9.998 0.005

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.4. and values for the Tianheban Formation sample raw corr s d l^ c raw ^*0 coir s d l% PDB PDB PDBPDB T-59 1.737 0.984 0.003 -7.758 -7.923 0.009 T-58 2.01 1.257 0.002 -7.421 -7.586 0.005 T-56 1.677 0.924 0.003 -7.864 -8.029 0.005 T-54 1.439 0.686 0.005 -7.908 -8.073 0.005 T-52 1.41 0.554 0.004 -7.851 -8.053 0.009 T-51 1.146 0.373 0.002 -8.002 -8.156 0.007 T-50 2.013 1.157 0.005 -7.098 -7.3 0.006 T-50b 2.304 1.448 0.003 -6.715 -6.917 0.005 T-49 1.124 0.268 0.003 -8.312 -8.514 0.005 T-47 -0.854 0.243 0.004 -12.379 -7.883 0.008 T-46 -0.747 0.35 0.002 -12.206 -7.71 0.008 T-45 -0.719 0.378 0.004 -12.169 -7.673 0.007 T-43 0.77 -0.086 0.002 -7.465 -7.667 0.004 T-41 1.152 0.296 0.002 -8.108 -8.31 0.008 T-40 0.704 -0.152 0.003 -6.450 -6.652 0.006 T-38 1.469 0.696 0.003 -7.443 -7.597 0.002 T-35 1.759 0.986 0.005 -7.971 -8.125 0.007 T-33 1.51 0.654 0.002 -8.301 -8.503 0.004 T-32 1.068 0.295 0.003 -7.149 -7.303 0.006 T-30 -0.199 0.898 0.001 -12.361 -7.865 0.008 T-29 -0.064 1.033 0.003 -12.796 -8.3 0.01 T-28 -0.232 0.865 0.004 -12.711 -8.215 0.004 T-27 -0.619 0.478 0.002 -12.586 -8.09 0.005 T-25 -1.195 -0.098 0.003 -12.677 -8.181 0.008 T-24 -1.305 -0.208 0.003 -12.807 -8.311 0.003 T-23 -0.852 0.245 0.003 -12.66 -8.164 0.007 T-22 -0.188 0.909 0.003 -12.758 -8.262 0.004 T-21 0.124 1.221 0.003 -12.674 -8.178 0.01 T-21B 0.076 1.173 0.003 -12.264 -7.768 0.007 T-20 -0.3 0.797 0.003 -12.695 -8.199 0.006 T-19 -0.163 0.934 0.005 -12.384 -7.888 0.003 T-17 -0.675 0.422 0.004 -12.574 -8.078 0.01 T-16 -0.411 0.686 0.004 -12.328 -7.832 0.015 T-15 -0.806 0.291 0.004 -12.378 -7.882 0.006 T-14 -0.466 0.631 0.003 -12.238 -7.742 0.006 T-13 -0.179 0.918 0.005 -12.451 -7.955 0.008 T-12 -0.89 0.207 0.006 -13.313 -8.817 0.006 T-II -0.472 0.625 0.004 -12.915 -8.419 0.005 T-10 -0.401 0.696 0.003 -12.543 -8.047 0.004 T-9 -0.496 0.601 0.004 -13.431 -8.935 0.004 T-8 -0.253 0.844 0.002 -13.126 -8.63 0.008 T-7 -0.07 1.027 0.006 -13.189 -8.693 0.007 T-6 -0.1 0.997 0.003 -13.069 -8.573 0.009 T-5 -0.161 0.936 0.002 -13.237 -8.741 0.004 T-4 -0.781 0.316 0.003 -13.373 -8.877 0.008 T-3 -1.632 -0.535 0.003 -14.072 -9.576 0.006

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.5. and values for the Harkless Formation.

sample ^^Ciaw corr s d l^ c ^^Oraw corr sdl% 0 PDBPDB PDB PDB H-4 0.556 -0.197 1.149 -12.035 -12.2 0.006 H-3 -1.019 -1.772 0.003 -11.11 -11.275 0.012 H-2 -1.32 -2.073 0.005 -13.146 -13.311 0.008 H-1 -2.763 -3.516 0.002 -12.895 -13.06 0.005

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4

SYSTEMATIC AND MORPHOLOGIC TRENDS IN ARCHAEOCYATHS THROUGH

THE EARLY CAMBRIAN

Abstract

In order to better understand the mechanisms related to archaeocyathan reef

decline, archaeocyath extinction, and the 40 Ma metazoan reefless gap, we examined

archaeocyathan morphology and systematics from their peak reef building (mid-Early

Cambrian) through their decline (end-Early Cambrian) from western Nevada (Poleta and

Harkless formations of Nevada) and from the Yangtze Platform of China (Xiannudong

and Tianheban formations). The most distinct trend observed is the rapid faunal

changeover from predominately regular archaeocyaths to predominantly irregular

archaeocyaths by the end of the Early Cambrian. This trend is observed in both the strata

studied and in other previously published literature. Another distinctive trend involves

the apparent thickening of irregular intervallar elements through the Early Cambrian.

Neither the outer nor inner walls of the irregulars vary in thickness through time, but a

thickening is observed in the intervallum elements of the irregulars.

We suggest that both of these trends indicate that irregular archaeocyaths had a

physiological advantage over regulars, which allowed for irregulars to quickly out-

compete regulars towards the end of the Early Cambrian during a time of putative climate

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. change. We hypothesize this advantage to be the presence of photosymbionts, which

makes calcification more favorable even under conditions of reduced carbonate

saturation. Carbon mass-balance models indicate that atmospheric pCOz was rising in

the Early Cambrian, which would have decreased carbonate saturation state, inhibiting

calcification. Furthermore, during the Early Cambrian sea-surface temperatures are also

reported as rising. These two climatic variables are hypothesized to have contributed to

the decline of the archaeocyathan reef ecosystem and the 40 Ma metazoan reefless gap

that followed. No direct evidence has yet been observed that links climate change to

archaeocyathan decline, but we suggest that rising pCOz and sea surface temperatures

continues to be the most probable mechanism.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction

Reefs have existed since the Archean Eon. Archean and Proterozoic reefs were

built exclusively by microbes. However, in the Early Cambrian, the appearance of

archaeocyaths ushered in the beginnings of metazoan-calcimicrobial-built reefs.

Archaeocyaths are an extinct class of calcareous sponges, which are divided into two

informal groups, 'regulares' and 'irregulares' (Debrenne et al., 2002). These two groups

evolved at the same time, and they are typically found inhabiting the same reefs and

occupying the same ecological niches (Debrenne and Zhuravlev, 1992; Debrenne et al.,

2002). Irregulars are distinguished from regulars by their more complex morphologies

and their placement of living tissue within the skeleton (Debrenne and Zhuravlev, 1992;

Debrenne et al., 2002).

Archaeocyaths are known only from the Cambrian System (543- 490 Ma). They

first appeared in the Tommotian Stage of the Early Cambrian Epoch, exclusively on the

Siberian Platform. During the subsequent Early Cambrian stages they rapidly diversified

and dispersed globally, achieving their peak diversity in the mid-Early Cambrian

(Atdabanian/Botomian) (Fig. 4.1). In the late-Early Cambrian, archaeocyaths, although

still globally dispersed, declined in diversity and finally disappeared at about the end of

the Early Cambrian. Only two post-Early Cambrian occurrences of archaeocyaths have

been documented, one in the Middle Cambrian and another in the Upper Cambrian. Both

of these post-Early Cambrian occurrences are in Antarctica and both are irregular

archaeocyaths (Debrenne et al., 1984; Wood et al., 1992).

The global decline of archaeocyaths was relatively rapid (approximately 8 Ma),

but the nature of this decline and the mechanisms behind it are not well understood.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ma 488 Upper € 500 Middle € 510 — 511 - -Lgypnwp ] .Toyonlan Regression Botomian -S in s k Archaeocyaths Extinction 518— 1 Event A t d a b a n i a n Üî^f T o m m o t i a n m ü I I I I 10 Genera 542 Figure 4.1. Spindle diagram illustrating the number of archaeocyathan genera during the Early Cambrian. This diagram is modified from Rowland and Hicks (2004).

Reproduced witti permission of ttie copyrigfit owner. Furtfier reproduction profiibited witfiout permission. Zhuravlev and Wood (1996) argued that the Sinsk Event, which brought anoxic waters

onto the carbonate shelves, greatly reduced the generic diversity of sessile organisms in

the Botomian. This was followed by a global regression, the Hawke Bay Regression

(Palmer and James, 1979), in the Toyonian Stage, which is hypothesized to have led to

the extinction of the remaining archaeocyaths (Wood, 1999). These two events—the

Sinsk Event and the Hawke Bay Regression—doubtless took a heavy toll on

archaeocyathan habitat, but they were relatively brief phenomena, after which metazoan

reef ecosystems should have been able to recover within a few million years. However,

after the disappearance of archaeocyathan reefs at the end of the Early Cambrian, there

followed an extraordinarily long, 40 Ma interval during which metazoan reefs were

essentially nonexistent anywhere on Earth (Rowland and Shapiro, 2002). This suggests

that the disappearance of archaeocyathan reefs at the end of the Early Cambrian

represents a significant global deterioration in environmental conditions conducive for

the growth of metazoan reefs. Furthermore, it is well established that other

environmental changes were also occurring in the Early Cambrian, which may have

played a role in the disappearance of archaeocyaths and the 40 Ma interval that lacked

conspicuous metazoan-built reefs. Specifically, atmospheric CO 2 levels are inferred to

have been rising very rapidly in the Early Cambrian (Berner, 1991,1994; Berner and

Kothavala, 2001), along with rising sea surface seawater temperatures (Karhu and

Epstein, 1986).

In this study, we attempted to test the hypothesis that global environmental

changes, other than sea level change, contributed to the collapse of the Earth’s first

metazoan reef ecosystem and the 40 Ma hiatus in metazoan reef-building. In order to test

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that trends in archaeocyaths are global, we examined archaeocyathan generic diversity,

biomass, and morphologic variations in western North America and South China, from

the Atdabanian-Botomian to the Toyonian (approximately 10 Ma), using both field and

laboratory observations as well as data from the published literature. Because there are

not yet globally recognized stages for the Early Cambrian, we use Siberian stage

nomenclature (Fig. 4.1), which is widely used by archaeocyath workers.

Methods

One month of fieldwork in China was conducted in 2002. We measured and

sampled three sections of the Atdabanian/Botomian Xiarmudong Formation in southern

Shaanxi and northern Sichuan provinces, and one section of the Toyonian Tianheban

Formation in Hubei Province (Fig. 4.2). Fieldwork in southwestern U.S. was conducted

in 2003. We measured and sampled one section of the Atdabanian/Botomian Lower

Member of the Poleta Formation in Nevada, and four sections of the Toyonian Harkless

Formation, also in Nevada (Figs. 4.2,4.3). All samples were thin sectioned for

taxonomic, morphological, and paleoecological analyses.

In thin section, archaeocyaths were identified to the genus level. Species were not

distinguished due to the problematic nature of archaeocyathan species identification.

While determining the generic diversity of archaeocyaths, we also measured the relative

biomass of regular verses irregular archaeocyaths. Relative biomass was measured by

counting individuals in each thin section, and tabulating the number of regulars and

irregulars. Seventy-nine, 3 x 2 inch thin sections were used in this part of the study. For

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. China

Y angbeR ^er Nanjiang 107"' Shaanxi Province Sichuan ProvincA Yichang • Kunming Hubei Province Shatan / Fucheng .Liantuo ¥ L Wangjiaping

Nanjiang, -.^Yichang a • Three Gorges 1 # - Dam 1 0km

JSkiT^ 1

Mount Jackson US ’almetto Ridge ^ 95 Mtns.

Lida N V 2 6 6 NV774 Stewart'sX Mill \ Magruder Mount » Mtn. Gold Point Dunfee

Ca Nv 10 km

Figure 4.2. Location maps of field areas. (A) Map of China showing the location of the three measured sections (stars). The three Xiannudong Formations sites are: F = Fucheng section; X = Xinchao section and S = Shatan section. The Tianheban Formation section is called Wangjiaping. (B) Location map of part of Esmeralda County, NV with the Poleta Formation, Stewart's Mill locality (star) and the Harkless Formations four sections (triangle) marked.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Laurentian Chinese Strata Chinese Stages Siberian Great Basin Stages and N. Sichuan-S. Shaanxi and Zones Stages Strata Zones West East Mule Spring Megapalaeolenus & Kongming- T ianheban Saline Valley Zone dong Fm . (equivalent) c Fm. m i E CD Palaeolenus I Zone I 1 O Is 511 Ma c Shipai H arkless 2 I Yanwanbian Fm. Fm . CO CD (U lU CD Drepanuroides a> 6 Fm. 0) (0 a t Zone c Î £ CD OQ o t— Yunnanaspis- £ c p Yiliangella Zone o I 0) m o> P oleta o Q. Malungia Zone 518 Ma F m . I X ia n n u d o nLiangshui- g Yunnano- & Nevadella Fm. Q. o > C c jing Zone ZJQ. cephalus M i Fm. Is IfD) Zone c •o ■S (0 ‘c Campito Ysunyi- CD Io Fm. discus CD Fallotaspis Guojiaba I Zone Fm. IParabadiella 520 Ma Figure 4.3. Correlation diagram for the upper Early Cambrian of South China, Laurentia, and the Siberian Platform. Formations are placed in Stages and biozones based on biostratigraphy (Rowland, 1978; Yuan and Zhang, 1981; Qin and Yuan, 1984; Zhang, 1989; Ye et al., 1997; Yuan et al., 2001; Zhu et al., 2001; Zhuravlev and Riding, 2001; Hollingsworth, 2004; Shergold and Cooper, 2004). Formations in bold, italic font are those included in this study

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the purpose of measuring the thickness of the outer walls, inner walls, and intervallum

elements, we used only the best-preserved samples from the seventy-nine thin sections.

Skeletal elements chosen for measurement are those that are free of encrusting

calcimicrobes, display minimum diagenetic alteration, and have distinct margins. Due to

the vagaries of preservation, it was not practical to employ a rigorous protocol to ensure

randomly selected samples; we measured as many well-preserved elements as possible

following the same procedure for inner walls, outer walls, and intervallum elements.

Thus, if any researcher bias inadvertently occurred, such bias should apply equally to all

of the elements measured. Furthermore, our initial hypothesis was that the elements

would become thinner over geologic time. This hypothesis was completely falsified by

the data. We are confident that the results presented in this paper are not artifacts of

researcher bias. To ensure that skeletal thickness variations were not ontogenetic,

measurements were made on both longitudinal and serial transverse sections. No

archaeocyath worker has ever documented ontogenetic variation in skeletal element

thickness. Based on our measurements, the thickness of a particular element did not

change over the length of the longitudinal section or from one serial transverse section to

another.

In addition to the samples that we collected from China and Nevada, we

conducted a literature review to compile a database on worldwide archaeocyathan

occurrences from the Tommotian to the Toyonian. This database includes information on

the formations, ages, archaeocyathan generic diversity, numbers of irregulars and

regulars, diversity of other organisms in association with the archaeocyaths, size of reefs

(if present), presence of branching archaeocyaths, percentage of calcimicrobes and their

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. identity, and type of sediments in association with the archaeocyaths and/or the reefs.

The database currently contains fifty-six entries. It is by no means complete and will

continue to be expanded in the future, but it added greatly to this study by providing data

on archaeocyathan generic diversity through time at localities throughout the world.

Because thickness of skeletal elements of archaeocyaths is not routinely reported, all

thickness data utilized were measured during this study.

The detailed paleoecology of each of the formations is described elsewhere

(Rowland, 1978; Rowland and Gangloff, 1988; Debrenne et al., 1991; Hicks, 2001; Hicks

and Rowland, in prep.). The Poleta, Harkless, and Tianheban formations all contain

varying-sized archaeocyathan-calcimicrobial reefs, while the Xiannudong Formation

contains small to massive microbial reefs with sparse, irregular archaeocyaths as dwellers

within the microbial reefs but not as constructors. In the Xiannudong Formation, most of

the archaeocyaths appear to have lived in inter-reef communities that were ripped-up and

redeposited during storm events.

Archaeocyaths

Archaeocyathan morphology commonly consists of a highly porous, two-walled

skeleton of microgranular, high-Mg calcite (James and Klappa, 1983) (Fig. 4.4).

Skeletons vary from cup-shaped to laminar to disc-like, and they can be solitary or

branching. Past phylogenetic interpretations of archaeocyaths classified them as

coelenterates, sponges, algae, a phylum of their own (Phylum Archaeocyatha), and as

members of a separate kingdom (Kingdom Archaeata) (Rowland, 2001).

Archaeocyaths are now classified as an extinct class of aspiculate, calcareous

sponges. Class Archaeocyatha within Phylum Porifera, with two informal groups.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. m

Figure 4.4-Photomicrographs of transverse sections of the two types of archaeocyaths. (A) Irregular, Metadetes sp., with taeniae (T), outer wall (O), and inner wall (I) labled from the Harkless Formation. (B) Regular, RaseWcyathus, sp., with septa (S), outer wall (O), and inner wall (I) labled from the Xiannudong Formation.

100

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. regulars and irregulars. This classification has been adopted by all archaeocyathan

workers because of the close affinities in structure between archaeocyaths and some

living sponges such as spinctozoans and other demosponges, most notably the aspiculate

species Vaceletia crypta (Debrenne and Vacelet, 1984; Kruse and Debrenne, 1989;

Kruse, 1990a; Debrenne and Zhuravlev, 1992; Reitner, et al., 1997).

The two informal groups of archaeocyaths, regulars and irregulars, evolved at the

same time (Hill, 1972; Debrenne et al., 2002). The distinction between these groups is

based on the position of living tissue in the skeleton (Debrenne et al, 2002). In irregular

archaeocyaths, the living tissue grew upward as the skeleton grew (mobile aquiferous

system); lower portions of the skeleton were successively closed off as upper portions

were added. An additional diagnostic feature of irregular archaeocyaths is the presence

of wavy septa, called either taeniae or pseudo septa, which are commonly porous to

coarsely porous. The septa of regular archaeocyaths, in contrast to irregulars, are not

wavy; they may be porous, sparsely porous, or non-porous. In regulars, soft tissue

occupied the entire skeleton as the archaeocyath grew. Both irregulars and regulars occur

as solitary or branching colonies and are approximately the same size (Debrenne and

Zhuravlev, 1992).

In general, both irregulars and regulars may be found within the same reef, and

interactions between members of the two groups varied from passive to aggressive

(Debrenne and Zhuravlev, 1992). Typically, an aggressive relationship is inferred to

have existed between irregulars and regulars, with the former dominating and impeding

growth of the latter (Debrenne and Zhuravlev, 1992).

101

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Systematic and Morphologic Variations

Debrenne et al. (2002) recognize six orders of archaeocyaths (Fig. 4.5 A). All of

the orders except one, Monocyathida, can be assigned to either the regular type

(Ajacicyathida, Tabulacyathida, and Capsulocyathida) or the irregular type

(Archaeocyathida and Kazachstanicyathida). Monocyathida are archaeocyaths with only

one wall, so they are neither regulars nor irregulars. Figure 4.5 A shows the global

frequency through time of genera within each of the six orders. This plot was derived

using data from Debrenne et al. (2002). Figure 4.5B shows data derived using other

published literature in the newly created database in which the different orders are

grouped into regulars and irregulars (Appendix 1). Both sets of data, the Debrenne et al.

(2002) and our archaeocyathan database, were used to ensure that generic trends

illustrated were not just a product of subjective data acquisition.

In both graphs, the number of regular genera greatly exceeds the number of

irregular genera during the Atdabanian and Botomian (Figs. 4.5A, B). Regulars and

irregulars coincide in generic abundance only during the beginning of the Tommotian and

in the Toyonian.

Variations in Relative Biomass

In order to analyze for possible changes in the relative biomass of regular and

irregular archaeocyaths, we used samples from the Atdabanian-Botomian of China

(Xiannudong Formation) and Nevada (Poleta Formation) and also the Toyonian of China

(Tianheban Formation) and Nevada (Harkless Formation). Both the number of genera

and relative biomass of regular archaeocyaths were high in the Atdabanian-Botomian for

the Xiannudong and Poleta formations (Fig. 4.6). However, this changed dramatically in

102

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Generic Diversity of Archaeocyaths within Each Order

1 4 0 r 1 3 0 -^Monocyattiida 0 120 «ne» Ajacicyathida Tabulacyathida 2 110 Û> Capsulocyathida 100 Archaeocyathida Kazachstanicyathida

T1 T2 T3 T4 A1 A2 A3 A4 B1 B2 B3 Ty1 Ty2 Ty3 Early Cambrian Stages

Generic Diversity Using Literature Review 90, Regular 1 ^ » Irregular 70

0 5 0

« 4 0

1 30

^ 2 0

Atd/Bot BotomianAtdabanian Toyonian Siberian Stages Figure 4.5. Global frequency of archaeocyathan genera. (A) Frequency of genera in each of the six orders at different times within the Early Cambrian. Data are from Systema Porifera (Debreime et al., 2002). (B) Frequency of genera of regulars and irregulars at different times in the Early Cambrian. Data are from published literature review (Appendix 1). Tl-T4= Tommotian, Al-A4= Atdabanian, Bl-B3= Botomian, Tyl-Ty3= Toyonian.

103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. irregulars

Regulars

CO 100

Irreaulars

egulars

Atdabanian-Botomian I Toyonian archaeo<^ath type

Atdabanian-Botomian Toyonian Lower Xiannudong Tlanhet)an Harkless Poleta Number of thin 33 14 13 19 sections examined Total no. of irregular 55 13 76 66 archaeocyaths Total no. of regular 86 27 0 0 archaeocyaths Percent irregular 39% 32% 100% 100%

Percent regular 61% 68% 0% 0%

Figure 4.6. Relative Biomass of regular and irregular archaeocyaths from the Atdabanian-Botomian and the Toyonian. Atdabanian-Botomain histogram bars are combined data from the Xiannudong and Poleta formations. Toyonian histogram bar represents the combined data from the Tianheban and Harkless formations.

104

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the Toyonian, during which time the irregulars completely dominated. Only a single

skeleton of a regular archaeocyath was documented in the Harkless Formation, which

was observed only in outcrop (Hicks, 2001), and we found no regulars at all in the

Tianheban. This dominance of irregulars in Toyonian reefs has also been documented in

the well-studied Forteau Formation of Labrador, Canada (Debrenne and James, 1981;

James and Klappa, 1983).

Even more interesting is the faunal changeover observed within the Xiannudong

Formation itself. Within all three sections of the Xiannudong Formation examined in this

study, archaeocyath-bearing units change from being dominated by regulars in the lower

part to being dominated by irregulars in the upper part. Furthermore, our review of the

published literature indicates that this progressive dominance of irregulars over regulars

toward the end of the Early Cambrian is a global phenomenon. Even though the number

of genera of regulars is greater than the number of genera of irregulars (Figs. 4.5A, B),

the irregulars dominate in terms of biomass.

Thickness Variations of Skeletal Elements

In order to test the hypothesis that changing levels of atmospheric CO 2 and/or

rising sea surface temperature were influencing archaeocyathan-reef development, we

measured the thickness of selected skeletal elements of archaeocyaths in each of the four

formations (Table 4.1). With this hypothesis, we expect to see decreasing thickness of

skeletal elements in archaeocyaths due to the lowered pH in the surface seawater, which

is associated with increased atmospheric CO 2 (Buddemeier et al., 1998; Gattuso et al.,

1998; Langdon et al., 2000; Kleypas and Langdon, 2000).

105

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4.1 Skeletal thickness measurements from each of the localities in this study. Ir = irregular archaeocyath, R - regular archaeocyath O.W. thickness I.W. thickness Taniae or Septa Location Genus Tvpe Age ftto) fttn l thickness(pm) Poleta Fm Protonharetra h- Botomian 150.0 75.0 150.0 U n lD re g R 75.0 75.0 75.0 Ethmophvllum R 200.0 250.0 150.0 Ethmoohvllum R 100.0 125.0 100.0 Protopharetra Ir 100.0 125.0 100.0 ?Protooharetra Ir 125.0 100.0 150.0 ?Rotundocvathus R 100.0 75.0 50.0 U n ID rea R 50.0 100.0 50.0 Protopharetra Ir 125.0 50.0 Location and sample number Xiannudong Fm X-F-1-7-7-B ?Spirocvathella Ir 150.0 75.0 50.0 X-F-1-7-7-B ?Spirocvathella Ir 75.0 100.0 50.0 X-F-2-7-7 Graphoscvphia Ir 50.0 75.0 25.0 X-F-3-7-7 Clathrocoscinus R 75.0 75.0 25.0 X-F-3-7-7 Spirocvathella? Ir 100.0 100.0 50.0 X-F-3-7-7 Graphoscvphia Ir 75.0 50.0 37.5 X-F-9-7-7 Protopharetra Ir 125.0 125.0 100.0 X-F-1-7-8 UnlDreg R 25.0 50.0 50.0 X-F-6-7-8b2 Inessocvathus R 37.5 50.0 25.0 X-F-8-7-8c Protopharetra Ir 75.0 50.0 75.0 TConannulofugia X-F-8-7-8b TTaylorcyalhus R 50.0 50.0 25.0 X-F-4-7-9 bot Pilodicoscinus R 37.5 50.0 50.0 X-F-4-7-9 Pilodicoscinus R 50.0 75.0 25.0 X-F-4-7-9 Coscinocvathus R 75.0 50.0 50.0 X-F-4-7-9 Changicvathus Ir 125.0 125.0 25.0 X-F-4-7-9 Dictvocvathus Ir 100.0 0.0 50.0 X-F-4-7-9 Tavlorcvathus R 75.0 25.0 25.0 X-F-5-7-9 Coscinocvathus R 100.0 175.0 50.0 X-F-4-7-10 Inessocvathus R 75.0 100.0 50.0 X-F-4-7-10 Protopharetra Ir 100.0 100.0 75.0 X-F-4-7-10 Protopharetra Ir 75.0 50.0 50.0 X-F-4-7-10 Protopharetra Ir 175.0 50.0 X-X-5b-7-ll Protopharetra Ir 125.0 100.0 50.0 X-X-9C-7-11 Coscinocvathus R 50.0 50.0 25.0 X-X-9b-7-ll Coscinocvathus R 75.0 37.5 25.0 X-X-9b-7-ll Rassetticvathus R 50.0 25.0 25.0 X-X-9b-7-ll Rassetticvathus R 37.5 37.5 25.0 X-X-5b-7-ll Graphoscvphia Ir 125.0 75.0 50.0 X-X-5b-7-ll ?Spirocvathella Ir 125.0 75.0 50.0 X-X-9a-7-ll Inessocvathus R 25.0 75.0 25.0 X-X-9a-7-ll Rassetticvathus R 50.0 50.0 25.0 X-X-11-7-11 Clathrocoscinus R 50.0 100.0 50.0 Location and sample number Tianheban Fm. T8c Archaeocvathus Ir Tovonian 100.0 125.0 T8c Archaeocvathus Ir 125.0 300.0 125.0 5b Archaeocvathus Ir 75.0 125.0 100.0 5b Archaeocvathus Ir 125.0 75.0 125.0 T4f Archaeocvathus Ir 150.0 175.0 250.0 1A4 Archaeocvathus Ir 75.0 112.5 100.0 lA l Archaeocvathus Ir 200.0 125.0 112.5 lA l Archaeocvathus Ir 87.5 50.0 100.0 lA l Archaeocvathus Ir 150.0 175.0 250.0 T 4f Archaeocvathus Ir 150.0 250.0 200.0 5b Archaeocvathus Ir 150.0 75.0 125.0 T8c Archaeocvathus Ir 100.0 250.0 125.0 T8c Archaeocvathus Ir 100.0 125.0 Location and sampk number Haridess Fm. S1-D17B2 Metacvathellus Ir 125.0 150.0 125.0 S1-D17-B2 Metaldetes Ir 125.0 175.0 125.0 S1-D17-B Metacvathellus Ir 125.0 125.0 137.5 S1-D17-B Metacvathellus Ir 125.0 100.0 137.5 S2-F5 Archaeosvcon Ir 175.0 200.0 175.0 Sl-D13c2 Archaeocvathus Ir 100.0 75.0 125.0 Sl-D 13c2 Archaeocvathus Ir 125.0 100.0 75.0 Sl-D 13c2 Archaeocvathus Ir 100.0 75.0 112.5

106

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4.1 ontinued. Skeletal thickness measurements of irregular archaeocyaths.

O.W. thickne I.W. thicknes Taniae or Septa thickn Location Genus Tvpe Age O tt) (Bn) (Bn) Harkless Fm. S1-D12C Archaeocvathus Ir 100.0 100.0 75.0 Sl-D8a Metacvathellus Ir 125.0 150.0 100.0 Sl-D 7a Archaeocvathus Ir 125.0 75.0 75.0 Sl-D7a Metacvathellus Ir 150.0 150.0 125.0 Sl-D7a Metacvathellus Ir 150.0 125.0 125.0 Sl-D7a Archaeocvathus Ir 125.0 75.0 75.0 Sl-D IO B I Retilamina Ir 112.5 75.0 50.0 Sl-D IO B I Archaeocvathus Ir 100.0 75.0 75.0 S l-D lO B l Retilamina Ir 125.0 125.0 100.0 Sl-D lO B l Metacvathellus Ir 175.0 150.0 125.0 S1-D10B1 Archaeocvathus Ir 150.0 75.0 87.5 Sl-D IO B I Retilamina Ir 62.5 0.0 75.0 S1-D15C Archaeocvathus Ir 100.0 100.0 100.0 Isotone sample Retilamina Ir 125.0 87.5 87.5

107

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. However, from the Atdabanian-Botomian to the Toyonian, the intervallum

elements of irregular archaeocyaths increased in thickness (Fig. 4.7 A). Meanwhile, the

outer walls and inner walls remained relatively unchanged (Fig. 4.7B, C, Table 4.2). The

mean intervallum elemental thickness in the Atdabanian-Botomian is 65 jtm, while the

mean intervallum elemental thickness in the Toyonian is 141 |i,m.

Graphs of Figure 4.8A-F show that the thickness of skeletal elements that are of

the same age did not significantly vary between the two localities of

Atdabanian/Botomian archaeocyaths and the two localities of Toyonian archaeocyaths.

Because no regular archaeocyaths are present in the Tianheban, and only a single

specimen of a regular was observed in the Harkless, we are unable to test whether the

intervallum elements of regulars increased in thickness. Toyonian regulars have been

described in Australia (Kruse, 1990b; Brock and Cooper, 1993), Antarctica (Debrenne

and Kruse, 1989), and Greenland (Debrenne and Peel, 1986), but no data are available on

the thickness of the skeletal elements.

Mechanisms for Systematic Change

Two trends are apparent in the data: (1) the relative biomass of irregulars

increased from the Atdabanian-Botomian though the Toyonian, and (2) the intervallum

elements of irregular archaeocyaths thickened through the Early Cambrian. Two

mechanisms, which are coupled, are hypothesized as the forcing agents of these

systematic and morphologic variations in archaeocyaths: rising atmospheric CO; and

rising sea surface temperatures. Berner (1991,1994) and Berner and Kothavala (2001)

have used a carbon-mass-balance approach to model atmospheric CO; variations for the

108

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Intervallum element thickness means of irregulars between Atdabanian/Botomian and Toyonian

140-,

120-

40- Intervallum Atd/Bot Intervallum Toy

limer wall thickness means of irregulars between Atdabanian/Botomian and Tovonian

5 100-

IW Atd/Bot

Outer wall thickness means of imegulars

140.

130-

^ 120- a

® 110. 5

100-

90-

OWAtb/bot OWToy Figure 4.7. Thickness means (mm) of inner wall (IW), outer wall (OW), and intervallum elements of irregular archaeocyaths. Bars represent variance around the mean. Data is from the four formations in this study. Atd/Bot = Atdabanian/Botomian, Toy = Toyonian

109

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4.2 Averages, Minimums (Min.), Maximums (Max ), Medians, and Modes of measured thicknesses of the Atdabanian/Botomian (A/B) and Toyonian regular and irregular archaeocyath outer walls, inner walls, and intervallum elements (Interval ). Formation names are simplified to P (Poleta Formation), X (Xiannudong Formation), H (Harkless Formation), and T (Tianheban Formation). The sample size (n) for each test is noted. A nalyses A/B Toyonian A/B Toyonian A/B T oyonian Outer w all Outer w all Inner w all Inner w all Interval. Interval. Average 106 pm 123 pm 94 pm 149 pm 65 pm 141 pm M in. 15 pm 20 pm 50 pm 50 pm 25 pm 100 pm M ax. 50 pm 75 pm 175 pm 300 pm 150 pm 250 pm M edian 100 pm 125 pm 100 pm 125 pm 50 pm 125 pm M ode 125 pm 125 pm 75 pm 125 pm 50 pm 125 pm

110

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mean thickness of inner walls for Mean thickness of inner walls for irtegluars irregulars from the Poleta and Xiannudoiig

^ 2 2 0 -

2 0 0 -

1 8 0 -

E 3 - 1 6 0 - c

8 0 - IW Poleta IW Xiannudong IW Harkless IW Tianheban Mean thickness of outer walls for Mean thicknessa of outer walls of irregulars irregulars from the Poleta and Xiannudong m 1 6 0 - H 1 5 0 -

1 4 0 -

„ 1 3 0 -

I, 120- 1 1 2 0 - 5

100- 1 1 0 -

1 0 0 -

OW Poleta OW Xianndong OW Harkless OWTiahheban Mean thickness of intervallum elements of Mean tbickiKss of intervallum elements of

|C1 1 8 0 - [ p ] 1 8 0 -

1 6 0 - 1 6 0 -

1 4 0 - <1 „ 1 4 0 . 'e 1 2 0 - E c m 1 1 0 - g 1 2 0 - 5 5 8 0 - T ^ 1 0 0 - 6 0 - 1 4 0 - I 8 0 - 1 ■ Intervallum P Intervallum X Intervallum H Intervallum T Figure 4.8. Thickness means (mm) of inner wall (IW), outer wall (OW), and intervallum elements of irregular archaeocyaths. Bars indicate variance around the mean. Data is from the four formations in this study. Atd/Bot = Atdabanian/ Botomian, Toy = Toyonian

111

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. entire Phanerozoic (Fig. 4.9). Other atmospheric CO 2 models (Tajika, 1998; Wallmann,

2001; and Kashiwagi and Shikazono, 2003) do not extend to the Early Phanerozoic, but,

for younger periods of time, these other models show considerable agreement with the

GEOCARB Models of Berner. GEOCARB Models 1-3 all show rapidly rising

atmospheric CO2 during the Early Cambrian (Fig. 4.9) (Berner, 1991, 1994; Berner and

Kothavala, 2001; Royer et al., 2004).

Numerous studies have shown that rising atmospheric CO 2 causes a

corresponding rise in ocean surface water CO 2 (CO2 aqueous) (Gattuso et al., 1998;

Gattuso et al., 1999; Kleypas and Langdon, 2000; Langdon et al., 2000; Leclercq et al.,

2000; Kleypas et al., 2 0 0 1 ; Feely et al., 2004). This rise in aqueous CO 2 causes a

lowering of pH and the carbonate saturation state, in which COs^ is the limiting factor in

calcium carbonate precipitation (Buddemeier et al., 1998; Gattuso et al., 1998; Langdon

et al., 2000; Kleypas and Langdon, 2000) (Fig. 4.10). There is a positive correlation

between the carbonate saturation state and biogenic carbonate precipitation; as the

carbonate saturation state declines, the amount of biogenetic carbonate precipitation also

declines (Smith and Buddemeier, 1992; Gattuso et al, 1998; Gattuso and Buddemeier,

2000; Langdon et al., 2000; Stoll et al, 2002). This influence of carbonate saturation

state on biogenic carbonate precipitation has been observed in many different taxa,

including corals and foraminifera (Gattuso et al, 1999; Gattuso and Buddemeier, 2000;

Feely et al., 2004).

Conversely, calcification is enhanced by rising atmospheric temperatures that

accompany rising levels of CO 2 (Riding, 1996; Gattuso et al., 1999; Kleypas and

Langdon, 2000; Anderson et al, 2003; Riding and Liang, 2005). Increased temperature

112

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 -

(N 15-

10 -

MesozoicPaleozoic CEN

600 500 400 300 200 100 0 Age (m.y.) Figure 4.9. Berner’s atmospheric CO^ versus time graph (Berner, 1994). RCO^ is the ratio of atmospheric CO, in the past to the present. RCO 2 is calculated from the long term carbon cycle and other proxies, such as weathering rates, buri^, and Sr composition of seawater. The upper and lower dashed lines represent the range of error, while the bold black line represents the best fit estimate of the atmospheric COj.

113

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Increasing CO2 Increased Atmospheric Temperatures

surface ocean water

2 - CO2 aq CO2 + H2O ► H2 CO3 2CO3 +CO3 2 HCO 3 (CO2 + H2O) (Carbonic acid

Sequestor /------g:------\ Reduces CO3 availability to ^C02 + H2 O "^CH 20 +O 2 combine with Ca to form photosynthesis s______CaCOs >

Figure 4.10. Diagram illustrates the complex chemical reactions that occur hetween the atmosphere and surface sea water. CO 2 rising in the atmosphere causes increased atmospheric temperatures, increased sea surface temper atures, and increased aqueous CO 2 . CO2 combines with H 2 O to form 2 - carbonic acid. The carbonic acid is neutralized by combining with CO 3 to form bicarbonate. This reduces the availability of COg^' to combine with C to form biogenetic calcite. Through photosynthesis sequestoring CO 2 , the availability of COg^" increases, thus increasing the carbonate consentration.

114

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lowers the solubility constant of carbonate, thereby making it metabolically easier for the

organism to calcify (Riding, 1996; Gattuso et al., 1998; Gattuso et al., 1999). Only one

study has attempted to reconstruct surface water temperatures for the Phanerozoic as far

back as the Cambrian. Using oxygen isotopes from chert and phosphate pairs, biogenic

phosphate, and carbonate fossils, Karhu and Epstein (1986) concluded that sea surface

temperatures in the Cambrian ranged from 55.0 °C to 80.0 °C. While the exact

temperatures inferred by Karhu and Epstein (1986) have not been widely accepted, there

is a general agreement that the Cambrian was a time of transition from icehouse to

greenhouse conditions, and that by the end of the Early Cambrian temperatures were

much warmer than at present (Rowland and Shaprio, 2002).

Therefore, with increased temperatures, archaeocyaths should have been able to

precipitate CaCOs more easily, but an overall increase in skeletal thickness is not

observed because the carbonate saturation state is low. The initial hypothesis of this

study was that rising levels of atmospheric CO; and associated factors influenced the

decline and collapse of Early Cambrian archaeocyathan reef ecosystems. Therefore, the

expected observation was that archaeocyathan skeletons would get thinner prior to the

virtual extinction of archaeocyaths. Instead, we observe the virtual disappearance of

regular archaeocyaths, an expansion of irregulars, and a thickening of the intervallum

elements of the irregulars.

Both regulars and irregulars coexist in the same reefs for approximately 10

million years (Tommotian-Botomian) with regulars dominating the reef biomass.

However, as observed in the uppermost part of the Xiannudong Formation, irregulars

quickly replace regulars and dominate both reef and non-reef environments globally

115

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. during the late-Early Cambrian (Appendix 1). The rapid faunal changeover implies that

irregulars had a physiological edge over irregulars, and that regulars and irregulars may

have responded differently to global environmental changes that were occurring during

the end of the Early Cambrian.

Our new working hypothesis is that the key to the success of the irregulars was

photosymbiosis. This hypothesis is based on several lines of evidence: (1) almost all

modem sponges have symbionts and many species of reef-dwelling sponges have

photosymbionts (Wilkinson, 1987), (2) photosynthesis is known to enhance calcification

even with low carbonate saturation states (Smith and Buddemeier, 1992; Opdyke and

Wilkinson, 1993; Westbroek et al., 1994; Gattuso et al., 1999; Gattuso and Buddemeier,

2000; Arp et al., 2001), and (3) photosymbiosis has already been hypothesized to have

been present in archaeocyaths on the basis of morphological and paleoecological

arguments (Cowen, 1983, 1988; Rowland and Gangloff, 1988; Rowland and Shapiro,

2002).

Photosymbiosis utilizes CO 2 , thereby lowering H2 CO3 (carbonic acid) formation,

and increases the alkalinity and the carbonate saturation state (Opdyke and Wilkinson,

1993; Westbroek et al., 1994; Gattuso et al., 1999; Gattuso and Buddemeier, 2000;

Langdon et al., 2000; Arp et al., 2001) (Fig. 4.10). Experiments with living organisms

show that as photosymbionts intake CO 2 , the partial pressure of extracellular CO 2

decreases, and the carbonate saturation state increases, allowing for CaCOs precipitation

(Gattuso et al., 1999). There is no compelling reason to suggest that archaeocyaths did

not have endosymbionts, especially since Vaceletia, which is described as having close

affinities to archaeocyaths, houses numerous symbiontic bacteria within the mesohyle

116

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (middle tissue) which for archaeocyaths would be found in the intervallum (Reitner et al.,

1997).

For anatomic reasons, irregular archaeocyaths were more likely to have housed

photosymbionts than were the regulars. In contrast to regulars, irregulars contain

intervallum elements with large surfaces areas, which allowed for greater tissue volume.

Greater tissue volume, in turn, would have allowed for a greater amount of

endosymbionts. Possibly the regulars harbored no photosymbionts at all As the CO 2

increased in the Early Cambrian, irregulars containing photosymbionts would have been

able to continue to calcify despite the reduced carbonate saturation state, while those that

did not house photosymbionts, such as regulars, would have declined. We suggest that

the thickening of only the intervallum elements rather than all skeletal elements was due

to the location of the photosymbionts, within the intervallum of the irregulars. As the

photosymbionts utilized CO 2 , the immediate area surrounding the photosymbionts would

have become more saturated with respect to the carbonate ion, allowing for rapid

precipitation in that area.

Summary and Conclusions

During the last few million years of the Early Cambrian Epoch, several trends are

apparent; (1) the number of genera of both regular and irregular archaeocyaths declined

dramatically, (2) the relative abundance of regulars and irregulars changes from being

dominated by regular to being dominated by irregular from the Atdabanian-Botomian to

the Toyonian, and (3) the intervallum elements of irregular archaeocyaths become

conspicuously thicker. We propose that all of these phenomena are related.

117

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Sinsk Event probably affected the generic diversity of both types of

archaeocyaths, however, the Sinsk Event is not well establish globally and is not

recognized on Laurentia (Zhuravlev and Wood, 1996). Therefore, the Sinsk event is

unlikely to be the overarching cause for the decline in archaeocyathan genera during the

Atdabanian-Botomian. Moreover, the forty-million-year-long interval without metazoan

reefs, which followed the collapse of the archaeocyathan reefs, suggest that other, long

term environmental factors were at work in the Early Cambrian.

Based on our data, regular archaeocyaths were unable to adapt to changing

environmental conditions; and thereby, they became very scarce toward the end of the

Early Cambrian. In contrast, the irregulars, whose morphology was more suited for

harboring photosynthesizing endosymbionts, were able to thrive for a while and

conspicuously out-compete the regulars.

We propose that rising levels of atmospheric CO2 and attendant increases in sea

surface temperatures provide the simplest explanation for archaeocyathan decline and the

40 Ma metazoan reefless gap. Direct evidence supporting this hypothesis is not yet

evident. However, high temperatures are suspected to cause coral bleaching, suggesting

that too great a temperature causes excessive thermal stress on an organism, leading to

death (Done, 1999; Kleypas et al., 2001; Hughes et al, 2003). No long-term studies have

been conducted on organisms’ responses to long-term temperature rise nor rapid rising

and prolonged high atmospheric CO 2

The proximal cause of the collapse of the Early Cambrian archaeocyathan-

calcimicrobial reef ecosystem was probably loss of habitat associated the Toyonian

Regression (Rowland and Gangloff, 1988; Wood, 1999), but if conditions had been

118

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. suitable, archaeocyaths or other metazoans should have hackstepped off the carbonate

platforms to keep pace with sea level fall or evolved into new niches in the Middle

Cambrian. The inability of archaeocyaths to reestablish themselves as reef-builders, or

any other metazoan to fill the reef-building niche, must be due to unsuitable, long-term

environmental conditions, such as rising atmospheric CO 2 and high sea surface water

temperatures. As we get closer to understanding the past climate, with respect to

atmospheric CO2 and global temperatures, we may be able to use the physical reactions

observed in archaeocyaths to evaluate a threshold of atmospheric CO 2 and temperature

for which calcifying organisms cannot tolerate.

Acknowledgments

The American Museum of Natural History, Institute for Cambrian Studies, the

Paleontological Society, and Geological Society of America grants all contributed

monetarily to this research. We also want to thank reviewers at UNLV for their

insightful comments on the first draft.

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127

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTERS

SUMMARY OF MAJOR RESULTS

The research presented in the previous three papers represents an attempt to

reconstruct the complex and dynamic interactions between archaeocyaths, climate, and

other environmental factors. The original four hypotheses for this study were (1) the

generic richness of archaeocyaths declined from the mid-Early Cambrian to the end-Early

Cambrian, (2) the percentage of branching archaeocyaths decreased from the mid-Early

Cambrian to the end-Early Cambrian, (3) the percentage of irregulars increased from the

mid-Early Cambrian to the end-Early Cambrian, and (4) the Lower Poleta Formation can

be correlated to the Xiannudong Formation and the Harkless Formation can be correlated

to the Tianheban Formation using Ô^^C isotopes. Each of these four hypotheses were

tested, and they produced interesting and unexpected results that provide information on

the paleoecology and paleogeography of the Yangtze Platform, the use of

chemostratigraphy as a correlation tool, and the enigmatic decline of archaeocyaths.

Sedimentologic and paléontologie evidence indicates that during the mid-Early

Cambrian archaeocyaths were not forming reefs on the Yangtze Platform, although

archaeocyaths were constructing reefs in Laurentia. Instead, on the Yangtze Platform,

archaeocyaths formed inter-reefal communities between thrombolite and stromatolite

reefs in a shallow, subtidal setting. Adjacent to the microbial reefs and archaeocyathan

128

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. communities there were migrating ooid shoals that commonly shifted position and

covered the reefs during storm events. Such storm events also ripped-up archaeocyaths

and redeposited them as storm debris between the microbial reefs. These large storm

events occurred repeatedly throughout the deposition of the Xiannudong Formation.

Paleoecological analyses on the Yangtze Platform also indicate that, at least

during the Early Cambrian, this region was a carbonate platform that occupied tropical

latitudes. Deposits across the platform are similar to those observed in the Bahamas, and,

therefore, must have formed in a tropical environment. The presence of offshore, slope,

and basinal sediments on the southern and eastern parts of the platform (APC, 1985;

Zhang and Pratt, 1994; Zhu et al., 2001), indicate that the shallow, carbonate deposits

were surrounded by deeper water.

Because biostratigraphy did not prove useful for precisely correlating global

sections, I attempted to use chemostratigaphy and the composite ô^^C profile for the

Early Cambrian of the Siberian Platform (Brasier et al., 1994) to better constrain the

correlation between the Xiannudong and Poleta formations and the Tianheban and

Harkless formations. Surprisingly, all five sections analyzed for Ô^^C show no major

excursions, which is contrary to the ô^^C record for the Siberian Platform, and, therefore,

not useftil for correlative purposes. I hypothesize that Ô^^C values I collected in this

study may not record global Ô^^C variations, but rather record basinal variations; or the

ô^^C profile of the Siberian Platform may be unique to that particular platform and does

not represent the secular ô^^C record.

Throughout the paleoecological, sedimentological, and chemostratigraphic

analyses, I documented the generic diversity of archaeocyaths and the relative abundance

129

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of regular and irregular archaeocyaths from the mid-Early Cambrian to the late-Early

Cambrian. The trends observed are (1) decreasing generic diversity of archaeocyaths, (2)

increasing relative abundance of irregular archaeocyaths, and (3) increasing thickness of

certain skeletal elements of irregular archaeocyaths. Because a significant rise in

atmospheric CO; has been inferred for the Early Cambrian (Berner, 1991, 1994, 1997;

Berner and Kothavala, 2001), the increasing skeletal thickness of intervallar elements

was unexpected. This phenomenon is interpreted to record the presence of

photosymhionts in irregular archaeocyaths. Photosymbionts uptake carbon in the form of

CO2 , thereby lowering the carbonate saturation state. This counters the present paradigm

that photosymbiotic relationships did not evolve until the Upper Triassic (Wood, 1999).

The increasing relative abundance of irregular archaeocyaths during the Early

Cambrian while regular archaeocyaths decreased in abundance indicates that irregulars

must have had some selective edge over the regulars. Because irregulars have more

elements in their intervallum, they also contained more soft tissue in their intervallum,

and they could host more endosymbionts than could regulars. Furthermore, regular-type

archaeocyath may not have housed any endosymbionts. In the sections sampled in this

study, no regular archaeocyaths were present in the late-Early Cambrian; it was not

possible to determine whether similar thickness trends are also present in regulars.

This dissertation proposes many more questions than it answers, but it does create

new hypotheses for continued research. Future research should include more

stratigraphie analyses of the Yangtze Platform. More chemostratigraphic data need to be

collected from both Laurentia and China. These two regions still lack a standard ô^^C

curve, which can be used to interpret environmental change as well as a correlation tool

130

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. both basinally and, perhaps, globally. Finally, the faunal changeover from regular

archaeocyath dominance to irregular archaeocyath dominance should be explored in other

localities, such as Australia and Siberia. Only three late-Early Cambrian localities

contain regular archaeocyaths; Australia (Kruse, 1990b; Brock and Cooper, 1993),

Antarctica (Debrerme and Kruse, 1989), and Greenland (Debrenne and Peel, 1986).

These regulars should be analyzed to determine if any systematic variation is present in

the skeletons of regulars, as it is in irregulars.

Global decline of reefs significantly affects the Earth system. Modem reefs are

sinks for atmospheric carbon, peaks of biodiversity, barriers to shoreline erosion, possible

sources of pharmaceuticals, and magnets for ecological tourism. By understanding how

Early Cambrian reef ecosystems responded to rising levels of CO 2 , we can better predict

how anthropogenic CO 2 and associated climate change will affect modem reefs. The past

is a key to the present, and the Early Cambrian maybe a key for understanding the

Holocene.

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Bemer, R , and Kothavala, Z , 2001, GEOCARB HI: a revised model of atmospheric CO 2

over Phanerozoic time: American Joumal of Science, v. 301, p. 182-204.

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1994, Multiple ô^^C excursions spanning the Cambrian explosion to the

Botomian crisis in Siberia: Geology, v. 22, p. 455-458.

Brock, G A, and Cooper, B. J., 1993, Shelly fossils from the Early Cambrian (Toyonian)

Wirrealpa, Aroona Creek, and Ramsay limestones of South Australia: Joumal of

Paleontology, v. 67, p. 758-787.

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Land, central North Greenland: Gronlands Geologiske Underogelse, v. 132,

p. 39-50.

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ed.. Origins and Evolution of the Antarctic Biota, Geological Society Special

Publication No. 47, p. 15-28.

Kruse, P., 1990b, Cyanobacterial—archaeocyathan—radiocyathan bioherms in the

Wirrealpa Limestone of South Australia: Canadian Joumal of Earth Science,

V. 28, p. 601-615.

Wood, R , 1999, Reef Evolution: Oxford, Oxford University Press, 414p.

Zhang, X. and Pratt, B , 1994. New and extraordinary Early Cambrian sponge spicule

assemblage from China. Geology 22, 43-46.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Zhu, Maoyan, Zhang, Junming, and Li Guoxiang, 2001. Sedimentary environments of the

Early Cambrian Chengjiang Biota: sedimentology of the Yu’Anshan Formation in

Chengjiang County, eastern Yunnan. Acta Palaeontologica Sinica 40, 80-105.

Zhuravlev, A. Yu., and Wood, R A , 1996, Anoxia as the cause of the mid-Early

Cambrian (Botomian) extinction event: Geology, v. 24, p. 311-314.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VITA

Graduate College University of Nevada, Las Vegas

Melissa Hicks

Local Address; 1362 Dorothy Ave. #4 Las Vegas, NV. 89119

Home Address: 405 Devon Terrace Shillington, PA. 19607

Degrees: Bachelor of Science, Geology, 1999 Juniata College, Huntingdon, PA.

Master of Sciences, Geology, 2001 University of Nevada, Las Vegas

Special Honors and Awards: Awarded the President’s Graduate Fellowship, 2004 UNLV GREAT Assistantship 2002 Best Masters Thesis Award, University of Nevada, Las Vegas, 2001 Arizona-Nevada Academy of Science Best Student Paper Award, 2001 University of Nevada, Las Vegas Graduate Student Association Forum, Best Poster or Talk, 2001, 2003, 2005 Dean’s List, Juniata College in 1998

Publications: Hicks, Melissa, 2006, A new genus of Early Cambrian coral in Esmeralda County, southwestern Nevada: Joumal of Paleontology, v. 80, no. 4, (in press)

Hicks M., 2002, Correlating Early Cambrian archaeocyathan-built reefs across North America and China: Stratigraphy of the Xiannudong Formation: Arizona-Nevada Academy of Science, Graduate Student Grant-in-Aid Award, 2002. Available from: httD://\YWw.&eo.arizona.edu/anas/av'/ards/mhick503.ndf

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rowland, S.M. and Hicks, M., (in review). Late Cambrian/Middle Cambrian/Early Cambrian/Ichnocambrian, submitted to Geology

Hicks, M. and Rowland, S.M. (in prep ). Systematic and morphologic trends in Early Cambrian archaeocyaths: tracking organism variations and climate change, for submittal to Palaios

Hicks, M. Rowland, S.M. (in prep), Paleoecologic study of the northern portion of the Yangtze Platform: Bahamas-type facies in the Early Cambrian, for submittal to Palaeogeography, Palaeoclimatology, Palaeoecology

Hicks, M. and Rowland, S.M., (in prep), Chemostratigraphic analysis of Early Cambrian microbial reefs, archaeocyathan reefs, and associated facies in Nevada and South China: surprising story of stasis, for submittal to the Joumal of Sedimentary Research

Field Guides and Short Courses

Anderson, Thomas B , Hicks, Melissa, Shaprio, Russell S., 2005, Microbialite sediments in the Death Valley Area, in Stevens, C , Cooper, J. eds., Westem Great Basin Geology: Joint meeting of the Cordilleran GSA and Pacific Section AAPG, Guidebook 99, p.67,90-107

Hicks, Melissa, 2005, Toyonian archaeocyathan-microbial reefs in the Upper part of the Harkless Formation, in Stevens, C , Cooper, J. eds., Westem Great Basin Geology. Joint meeting of the Cordilleran GSA and Pacific Section AAPG, Guidebook 99, p. 80-89.

Rowland, Stephen M. and Hicks, Melissa, 2004, Early Cambrian Experiment in reef building by Metazoans, in Lipps, J.H. and Waggoner, B.M., eds., Neoproterozoic- -Cambrian Biological Revolutions: The Paleontological Society Papers, v. 10, p. 107-130.

Abstracts

Hicks, Melissa and Rowland, Stephen, 2005, Do morphological and systematic trends in archaeocyaths record changing climate in the Early Cambrian?: Geological Society of America, Abstracts with Programs, v. 37, p. 135.

Hicks, Melissa and Rowland, Stephen, 2005, Analyses of the chronostratigraphy and the paleocommunity of strata fi'om Botomian through Toyonian of China and westem Laurentia: changing and declining benthic communities: Acta Micropalaeontologica Sinica, v. 22 (supplement), p. 58.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hicks, Melissa, 2005, Calcimicrobes and archaeocyaths in Early Cambrian Reefs: What do we really know about their relationship?: Geological Society of America, Joint meeting, Cordilleran Section/AAPG Pacific Section, v. 37, no. 4.

Hicks, Melissa and Rowland, Stephen, 2004, A paleoecological study of Early Cambrian stromatolitic, thromboUtic, and archaeocyathan reefs on the Yangtze Platform, southern Shaanxi Province, China: cycles, trends, and associations: Geological Society of America, Abstracts with Programs, v. 36, p. 57.

Mrozek, Stephanie, Dattilo, Benjamin F., Hicks, Melissa, and Miller, James F, 2003, Metazoan reefs fi'om the Upper Cambrian of the Arrow Canyon Range, Clark County, Nevada: Geological Society of America, Abstracts with Programs, V. 35, p. 500.

Hicks, M. and Rowland, S., 2003, Comparison of late-Early Cambrian archaeocyathan reefs from Nevada, U.S.A. and westem Hubei, China: Geological Society of America, Abstracts with Programs, v. 35, p. 500.

Hicks, M., 2002, Why did metazoan reefs disappear for forty million years in the Early Paleozoic?: Biological vs. physicochemical mechanisms: Geological Society of America, Abstracts with Programs, vol.34, p.65.

Hicks, Melissa and Shapiro, Russell, 2002, Development of a sequence stratigraphie framework for the Upper Nopah Formation (Upper Cambrian, Eastem California and Westem Nevada): Geological Society of America, Rocky Mountain, 54 Annual Meeting, Abstracts with Programs, vol.34, p.51.

Hicks, M. and Rowland, S., 2001, Paleoecology of Lower Cambrian archaeocyathan reefs in Esmeralda County, Nevada: Proceedings of the Arizona-Nevada Academy of Science, Forty Fifth Annual Meeting, 2000-2001 Annual Reports.

Hicks, M. and Rowland, S., 2001, The final episode of Early Cambrian metazoan reef- building in westem Laurentia: High-diversity archaeocyathan-calcimicrobe reefs in the Upper Harkless Formation of Esmeralda County, Nevada: North American Paleontological Convention, PaleoBios, v. 21, no. 2, p.66

Hicks, M.K, 2000, Paleoecology of Lower Cambrian archaeocyathan reefs in Esmeralda County, Nevada: influences on the collapse of metazoan reef ecosystems: American Association of Petroleum Geologists Bulletin, v.84, no. 11, p. 1864.

Hicks, M.K, and Rowland, S., 2000, Archaeocyathan-coralomorph reefs of the uppermost Lower Cambrian in the westem Great Basin, Nevada: Geological Society of America, Abstracts with Programs, v. 32, no. 7, p. 11-12.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hicks, M., Eastham, K., and Lehmann, D., 1999, Benthic community paleoecology along a lagoon-, shoal-, offshore transect: Middle Ordovician, Central Kentucky: Geological Society of America, Abstracts with Programs, v. 31, no.2, p. A-24.

Eastham, K., Hicks, M., Lehmann, D., 1999, High resolution stratigraphy of the Ordovician, Devils Hollow Member of the Lexington Limestone; lagoonal to offshore facies: Geological Society of America, Abstracts with Programs, v.31, no.2, p. 13-14.

Dissertation Title: Characterizing Global Archaeocyathan Reef Decline in the Early Cambrian: Evidence from Nevada and China

Thesis Examining Committee: Chairperson, Dr. Stephen M. Rowland, Ph.D. Committee Member, Dr. Andrew Hanson, Ph.D. Committee Member, Dr. Terry Spell, Ph.D. Committee Member, Dr. Ganqing Jiang, Ph.D. Graduate Faculty Representative, Dr. Peter Starkweather, Ph.D.

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